APRIL 25th 1982, DOWNING STREET: Announcement of the recapture of South Georgia, in the Falkland Islands.

Mrs Thatcher: Ladies and gentlemen, the Secretary of State for Defence has just come over to give me some very good news ...

Secretary of State: The message we have got is that British troops landed on South Georgia this afternoon, shortly after 4 pm London time ... The commander of the operation has sent the following message: ''Be pleased to inform Her Majesty that the White Ensign flies alongside the Union Jack in South Georgia. God save the Queen.''

Mrs Thatcher: Just rejoice at that news and congratulate our forces and the marines. Goodnight, gentlemen.

Mrs Thatcher turns towards the door of No 10 Downing Street.

Reporter: Are we going to declare war on Argentina, Mrs Thatcher?

Mrs Thatcher (pausing on her doorstep): Rejoice.

Picture first the street where she breathed her last. It is a quiet street, sedate, shaded by old trees: a street of tall houses, their facades smooth as white icing, their brickwork the colour of honey. Some are Georgian, flat-fronted. Others are Victorian, with gleaming bays. They are too big for modern households, and most of them have been cut up into flats. But this does not destroy their elegance of proportion, nor detract from the deep lustre of panelled front doors, brass-furnished and painted in navy or forest green. It is the neighbourhood's only drawback, that there are more cars than spaces to put them. The residents park nose-to-tail, flaunting their permits. Those who have driveways are often blocked into them. But they are patient householders, proud of their handsome street and willing to suffer to live there. Glancing up, you notice a fragile Georgian fanlight, or a warm scoop of terracotta tiling, or a glint of coloured glass. In spring, cherry trees toss extravagant flounces of blossom. When the wind strips the petals, they flurry in pink drifts and carpet the pavements, as if giants have held a wedding in the street. In summer, music floats from open windows: Vivaldi, Mozart, Bach.

The street itself describes a gentle curve, joining the main road as it flows out of town. The Holy Trinity church, islanded, is hung with garrison flags. Looking from a high window over the town (as I did that day of the killing) you feel the close presence of fortress and castle. Glance to your left, and the Round Tower looms into view, pressing itself against the panes. But on days of drizzle and drifting cloud the keep diminishes, like an amateur drawing half-erased. Its lines soften, its edges fade; it shrinks into the raw cold from the river, more like a shrouded mountain than a castle built for kings.

The houses on the right-hand side of Trinity Place – I mean, on the right-hand side as you face out of town – have large gardens, each now shared between three or four tenants. In the early 1980s, England had not succumbed to the smell of burning. The carbonised reek of the weekend barbecue was unknown, except in the riverside gin palaces of Maidenhead and Bray. Our gardens, though immaculately kept, saw little footfall; there were no children in the street, just young couples who had yet to breed and older couples who might, at most, open a door to let an evening party spill out on to a terrace. Through warm afternoons the lawns baked unattended, and cats curled snoozing in the crumbling topsoil of stone urns. In autumn, leaf-heaps composted themselves on sunken patios, and were shovelled up by irritated owners of basement flats. The winter rains soaked the shrubberies, with no one there to see.

But in the summer of 1983 this genteel corner, bypassed by shoppers and tourists, found itself a focus of national interest. Behind the gardens of No 20 and No 21 stood the grounds of a private hospital, a graceful pale building occupying a corner site. Three days before her assassination, the prime minister entered this hospital for minor eye surgery. Since then, the area had been dislocated. Strangers jostled residents. Newspapermen and TV crews blocked the street and parked without permission in driveways. You would see them trundle up and down Spinner's Walk trailing wires and lights, their gaze rolling towards the hospital gates on Clarence Road, their necks noosed by camera straps. Every few minutes they would coagulate in a mass of heaving combat jackets, as if to reassure each other that nothing was happening: but that it would happen, by and by. They waited, and while they waited they slurped orange juice from cartons and lager from cans; they ate, crumbs spilling down their fronts, soiled paper bags chucked into flowerbeds. The baker at the top of St Leonard's Road ran out of cheese rolls by 10 am and everything else by noon. Windsorians clustered on Trinity Place, shopping bags wedged on to low walls. We speculated on why we had this honour, and when she might go away.

Windsor's not what you think. It has an intelligentsia. Once you wind down from the castle to the bottom of Peascod Street, they are not all royalist lickspittles; and as you cross over the junction to St Leonard's Road, you might sniff out closet republicans. Still, it was cold comfort at the polls for the local socialists, and people murmured that it was a vote wasted; they had to show the strength of their feelings by tactical voting, and their spirit by attending outre events at the arts centre. Recently remodelled from the fire station, it was a place where self-published poets found a platform, and sour white wine was dispensed from boxes; on Saturday mornings there were classes in self-assertion, yoga and picture framing.

But when Mrs Thatcher came to visit, the dissidents took to the streets. They gathered in knots, inspecting the press corps and turning their shoul- ders to the hospital gates, where a row of precious parking bays were marked out and designated DOCTORS ONLY.

A woman said, ''I have a PhD, and I'm often tempted to park there.'' It was early, and her loaf was still warm from the baker; she snuggled it against her, like a pet. She said, ''There are some strong opinions flying about.''

''Mine is a dagger,'' I said, ''and it's flying straight to her heart.''

''Your sentiment,'' she said admiringly, ''is the strongest I've heard.''

''Well, I have to go in,'' I said. ''I'm expecting Mr Duggan to mend my boiler.''

''On a Saturday? Duggan? You're highly honoured. Better scoot. If you miss him he'll charge you. He's a shark, that man. But what can you do?'' She fished for a pen in the bottom of her bag. ''I'll give you my number.'' She wrote it on my bare arm, as neither of us had paper. ''Give me a ring. Do you ever go to the arts centre? We can get together over a glass of wine.''

Illustration by Henry McCausland.

I was putting my Perrier water in the fridge when the doorbell rang. I'd been thinking, we don't know it now, but we'll look back with fondness on the time Mrs Thatcher was here: new friendships formed in the street, chit-chat about plumbers whom we hold in common. On the entryphone there was the usual crackle, as if someone had set fire to the line. ''Come up, Mr Duggan,'' I said. It was as well to be respectful to him.

I lived on the third floor, the stairs were steep and Duggan was ponderous. So I was surprised at how soon I heard the tap at the door. ''Hello,'' I said. ''Did you manage to park your van?''

On the landing – or rather on the top step, as I was alone up there – stood a man in a cheap quilted jacket. My innocent thought was, here is Duggan's son. ''Boiler?'' I said.

''Right,'' he said.

He heaved himself in, with his boiler man's bag. We were nose to nose in the box-sized hall. His jacket, more than adequate to the English summer, took up the space between us. I edged backwards. ''What's up with it?'' he said.

''It groans and bangs. I know it's August, but –''

''No, you're right, you're right, you can never trust the weather. Rads hot?''

''In patches.''

''Air in your system,'' he said. ''While I'm waiting I'll bleed it. Might as well. If you've got a key.''

It was then that a suspicion struck me. Waiting, he said. Waiting for what? ''Are you a photographer?''

He didn't answer. He was patting himself down, searching his pockets, frowning.

''I was expecting a plumber. You shouldn't just walk in.''

''You opened the door.''

''Not to you. Anyway, I don't know why you bothered. You can't see the front gates from this side. You need to go out of here,'' I said pointedly, ''and turn left.''

''They say she's coming out the back way. It's a great place to get a shot.''

My bedroom had a perfect view of the hospital garden; anyone, by walking around the side of the house, could guess this.

''Who do you work for?'' I said.

''You don't need to know.''

''Perhaps not, but it would be polite to tell me.'' As I backed into the kitchen, he followed. The room was full of sunlight, and now I saw him clearly: a stocky man, thirties, unkempt, with a round friendly face and unruly hair. He dumped his bag on the table, and pulled off his jacket. His size diminished by half. ''Let's say I'm freelance.''

''Even so,'' I said, ''I should get a fee for the use of my premises. It's only fair.''

''You couldn't put a price on this,'' he said.

By his accent, he was from Liverpool. Far from Duggan, or Duggan's son. But then he hadn't spoken till he was in at the front door, so how could I have known? He could have been a plumber, I said to myself. I hadn't been a total fool; for the moment, self-respect was all that concerned me. Ask for identification, people advise, before letting a stranger in. But imagine the ruckus that Duggan would have caused, if you'd held his boy up on the stairs, impeding him from getting to the next boiler on his list, and shortening his plunder opportunities.

The kitchen window looked down over Trinity Place, now seething with people. If I craned my neck I could see a new police presence to my left, trotting up from the private gardens of Clarence Crescent. ''Have one of these?'' The visitor had found his cigarettes.

''No. And I'd rather you didn't.''

''Fair enough.'' He crushed the pack into his pocket, and pulled out a balled-up handkerchief. He stood back from the tall window, mopping his face; face and handkerchief were both crumpled and grey. Clearly it wasn't something he was used to, tricking himself into private houses. I was more annoyed with myself than with him. He had a living to make, and perhaps you couldn't blame him for pushing in, when some fool of a woman held the door open. I said, ''How long do you propose to stay?''

''She's expected in an hour.''

''Right.'' That accounted for it, the increased hum and buzz from the street. ''How do you know?''

''We've a girl on the inside. A nurse.''

I handed him two sheets of kitchen roll. ''Ta.'' He blotted his forehead. ''She's going to come out and the doctors and nurses are lining up, so she can appreciate them. She's going to walk along the line with her thank you and bye-bye, then toddle round the side, duck into a limo and she's away. Well, that's the idea. I don't have an exact time. So I thought if I was here early I could set up, have a look at the angles.''

''How much will you get for a good shot?''

''Life without parole,'' he said.

I laughed. ''It's not a crime.''

''That's my feeling.''

''It's a fair distance,'' I said. ''I mean, I know you have special lenses, and you're the only one up here, but don't you want a close-up?''

''Nah,'' he said. ''As long as I get a clear view, the distance is a doddle.''

He crumpled up the kitchen roll and looked around for the bin. I took the paper from him, he grunted, then applied himself to unstrapping his bag, a canvas holdall that I supposed would be as suitable for a photographer as for any tradesman. But one by one he took out metal parts, which, even in my ignorance, I knew were not part of a photographer's kit. He began to assemble them; his fingertips were delicate. As he worked he sang, almost under his breath, a little song from the football terraces:

You are a scouser, a dirty scouser,

You're only happy on giro day.

Your dad's out stealing, your mum's

drug-dealing,

Please don't take our hub-caps away.

''Three million unemployed,'' he said. ''Most of them live round our way. It wouldn't be a problem here, would it?''

''Oh no. Plenty of gift shops to employ everybody. Have you been up to the High Street?''

I thought of the tourist scrums pushing each other off the pavements, jostling for souvenir humbugs and wind-up Beefeaters. It could have been another country. No voices carried from the street below. Our man was humming, absorbed. I wondered if his song had a second verse. As he lifted each component from his bag he wiped it with a cloth that was cleaner than his handkerchief, handling it with gentle reverence, like an altar boy polishing the vessels for mass.

When the mechanism was assembled he held it out for my inspection. ''Folding stock,'' he said. ''That's the beauty of her. Fits in a cornflakes packet. They call her the widowmaker. Though not in this case. Poor bloody Denis, eh? He'll have to boil his own eggs from now on.''

It feels, in retrospect, as if hours stretched ahead, as we sat in the bedroom together, he on a folding chair near the sash window, his mug of tea cradled in his hands, the widowmaker at his feet; myself on the edge of the bed, over which I had hastily dragged the duvet to tidy it. He had brought his jacket from the kitchen; perhaps the pockets were crammed with assassin's requisites. When he flung it on the bed, it slid straight off again. I tried to grab it and my palm slid across the nylon; like a reptile, it seemed to have its own life. I flumped it on the bed beside me and took a grip on it by the collar. He looked on with mild approval.

He kept glancing at his watch, though he said he had no certain time. Once he rubbed its face with his palm, as if it might be fogged and concealing a different time underneath. He would check, from the corner of his eye, that I was still where I should be, my hands in view: as, he explained, he preferred them to be. Then he would fix his gaze on the lawns, the back fences. As if to be closer to his target, he rocked his chair forwards on its front legs.

I said, ''It's the fake femininity I can't stand, and the counterfeit voice. The way she boasts about her dad the grocer and what he taught her, but you know she would change it all if she could, and be born to rich people. It's the way she loves the rich, the way she worships them. It's her philistinism, her ignorance, and the way she revels in her ignorance. It's her lack of pity. Why does she need an eye operation? Is it because she can't cry?''

When the telephone rang, it made us both jump. I broke off what I was saying. ''Answer that,'' he said. ''It will be for me.''

It was hard for me to imagine the busy network of activity that lay behind the day's plans. ''Wait,'' I'd said to him, as I asked him, ''Tea or coffee?'' as I switched the kettle on. ''You know I was expecting the boiler man? I'm sure he'll be here soon.''

''Duggan?'' he said. ''Nah.''

''You know Duggan?''

''I know he won't be here.''

''What have you done to him?''

''Oh, for God's sake.'' He snorted. ''Why would we do anything? No need. He got the nod. We have pals all over the place.''

Pals. A pleasing word. Almost archaic. Dear God, I thought, Duggan an IRA man. Not that my visitor had named his affiliation, but I had spoken it loudly in my mind. The word, the initials, didn't cause me the shock or upset it would cause, perhaps, to you. I told him this, as I reached in the fridge for milk and waited for the kettle to boil: saying, I would deter you if I could, but it would only be out of fear for myself and what's going to happen to me after you've done it: which by the way is what? I am no friend of this woman, though I don't (I felt compelled to add) believe violence solves anything. But I would not betray you, because ...

''Yeah,'' he said. ''Everybody's got an Irish granny. It's no guarantee of anything at all. I'm here for your sightlines. I don't care about your affinities. Keep away from the front window and don't touch the phone, or I'll knock you dead. I don't care about the songs your bloody great-uncles used to sing on a Saturday night.''

I nodded. It was only what I'd thought myself. It was sentiment and no substance.

The minstrel boy to the war is gone,

In the ranks of death you'll find him.

His father's sword he has girded on,

And his wild harp slung behind him.

My great-uncles (and he was right about them) wouldn't have known a wild harp if it had sprung up and bitten their bottoms. Patriotism was only an excuse to get what they called pie-eyed, while their wives had tea and gingernuts then recited the rosary in the back kitchen. The whole thing was an excuse: why we are oppressed. Why we are sat here being oppressed, while people from other tribes are hauling themselves up by their own ungodly efforts and buying three-piece suites. While we are rooted here going la-la-la auld Ireland (because at this distance in time the words escape us) our neighbours are patching their quarrels, losing their origins and moving on, to modern, non-sectarian forms of stigma, expressed in modern songs: you are a scouser, a dirty scouser. I'm not, personally. But the north is all the same to southerners. And in Berkshire and the Home Counties, all causes are the same, all ideas for which a person might care to die: they are nuisances, a breach of the peace, and likely to hold up the traffic or delay the trains.

''You seem to know about me,'' I said. I sounded resentful.

''As much as anybody would need to know. That's to say, not that you're anything special. You can be a help if you want, and if you don't want, we can do accordingly.''

He spoke as if he had companions. He was only one man. But a bulky one, even without the jacket. Suppose I had been a true-blue Tory, or one of those devout souls who won't so much as crush an insect: I still wouldn't have tried anything tricky. As it was, he counted on me to be docile, or perhaps, despite his sneering, he trusted me to some small extent. Anyway, he let me follow him into the bedroom with my mug of tea. He carried his own tea in his left hand and his gun in his right. He left the roll of sticky tape and the handcuffs on the kitchen table, where he'd put them when they came out of his bag.

And now he let me pick up the phone extension from the bedside table, and hand it to him. I heard a woman's voice, young, timid and far away. You would not have thought she was in the hospital round the corner. ''Brendan?'' she said. I did not imagine that was his real name.

Margaret Thatcher in 1984. Photograph: PA Archive/PA Photos

He put down the receiver so hard it clattered. ''There's some friggin' hold-up. It'll be twenty minutes, she reckons. Or thirty, it could even be thirty.'' He let his breath out, as if he'd been holding it since he stomped upstairs. ''Bugger this. Where's the lav?''

You can surprise a person with affinity, I thought, and then say, ''Where's the lav?'' Not a Windsor expression. It wasn't really a question, either. The flat was so small that its layout was obvious. He took his weapon with him. I listened to him urinate. Run a tap. I heard splashing. I heard him come out, zipping his trousers. His face was red where he'd been towelling it. He sat down hard on the folding chair. There was a bleat from the fragile canework. He said, ''You've got a number written on your arm.''

''Yes.''

''What's it a number of?''

''A woman.'' I dabbed my forefinger with my tongue and slicked it across the ink.

''You won't get it off that way. You need to get some soap and give it a good scrub.''

''How kind of you to take an interest.''

''Have you wrote it down? Her number?''

''No.''

''Don't you want it?''

Only if I have a future, I thought. I wondered when it would be appropriate to ask.

''Make us another brew. And put sugar in it this time.''

''Oh,'' I said. I was flustered by a failing in hospitality. ''I didn't know you took sugar. I might not have white.''

''The bourgeoisie, eh?''

I was angry. ''You're not too proud to shoot out of my bourgeois sash window, are you?''

He lurched forward, hand groping for the gun. It wasn't to shoot me, though my heart leapt. He glared down into the gardens, tensing as if he were going to butt his head through the glass. He made a small, dissatisfied grunt, and sat down again. ''A bloody cat on the fence.''

''I have demerara,'' I said. ''I expect it tastes the same, when it's stirred in.''

''You wouldn't think of shouting out of the kitchen window, would you?'' he said. ''Or trying to bolt down the stairs?''

''What, after all I've said?''

''You think you're on my side?'' He was sweating again. ''You don't know my side. Believe me, you have no idea.''

It crossed my mind then he might not be a Provisional, but from one of the mad splinter groups you heard of. I was hardly in a position to quibble; the end result would be the same. But I said, ''Bourgeoisie, what sort of polytechnic expression is that?''

I was insulting him, and I meant to. For those of tender years, I should explain that polytechnics were institutes of higher education, for the young who missed university entrance: for those who were bright enough to say affinity, but still wore cheap nylon coats.

He frowned. ''Brew the tea.''

''I don't think you should sneer at my great-uncles for being cod-Irish, if you talk in slogans you found in skips.''

''It was a sort of a joke,'' he said.

''Oh. Well. Was it?'' I was taken aback. ''It looks as if I've no more sense of humour than she has.''

I indicated, with my head, the lawns outside the window, where the prime minister was shortly to die.

''I don't fault her for not laughing,'' he said. ''I won't fault her for that.''

''You should. It's why she can't see how ridiculous she is.''

''I wouldn't call her ridiculous,'' he said, mulish. ''Cruel, wicked, but not ridiculous. What's there to laugh at?''

''All things human laugh,'' I said.

After some thought, he replied, ''Jesus wept.''

He smirked. I saw he had relaxed, knowing that because of the friggin' delay he wouldn't have to murder yet. ''Mind you,'' I said, ''she'd probably laugh if she were here. She'd laugh because she despises us. Look at your anorak. She despises your anorak. Look at my hair. She despises my hair.''

He glanced up. He'd not looked at me before, not to see me; I was just the tea-maker. ''The way it just hangs there,'' I explained. ''Instead of being in corrugations. I ought to have it washed and set. It ought to go in graduated rollers, she knows where she is with that sort of hair. And I don't like the way she walks. 'Toddles', you said. She'll toddle round. You had it right, there.''

''What do you think this is about?'' he said.

''Ireland.''

He nodded. ''And I want you to understand that. I'm not shooting her because she doesn't like the opera. Or because you don't care for – what in sod's name do you call it? – her accessories. It's not about her handbag. It's not about her hairdo. It's about Ireland. Only Ireland, right?''

''Oh, I don't know,'' I said. ''You're a bit of a fake yourself, I think. You're no nearer the old country than I am. Your great-uncles didn't know the words either. So you might want supporting reasons. Adjuncts.''

''I was brought up in a tradition,'' he said. ''And look, it brings us here.'' He looked around, as if he didn't believe it: the crucial act of a dedicated life, ten minutes from now, with your back to a chipboard wardrobe glossed with white veneer; a pleated paper blind, an unmade bed, a strange woman, and your last tea with no sugar in it. ''I think of those boys on hunger strike,'' he said, ''the first of them dead almost two years to the day that she was first elected: did you know that? It took sixty-six days for Bobby to die. And nine other boys not far behind him. After you've starved yourself for about forty-five days they say it gets better. You stop dry-heaving and you can take water again. But that's your last chance, because after fifty days you can hardly see or hear. Your body digests itself. It eats itself in despair. You wonder she can't laugh? I see nothing to laugh at.''

''What can I say?'' I asked him. ''I agree with everything you've said. You go and make the tea and I'll sit here and mind the gun.''

For a moment, he seemed to consider it. ''You'd miss. You're not trained at all.''

''How are you trained?''

''Targets.''

''It's not like a live person. You might shoot the nurses. The doctors.''

''I might, at that.''

I heard his long, smoker's cough. ''Oh, right, the tea,'' I said. ''But you know another thing? They may have been blind at the end, but their eyes were open when they went into it. You can't force pity from a government like hers. Why would she negotiate? Why would you expect it? What's a dozen Irishmen to them? What's a hundred? All those people, they're capital punishers. They pretend to be modern, but leave them to themselves and they'd gouge eyes out in the public squares.''

''It might not be a bad thing,'' he said. ''Hanging. In some circumstances.''

I stared at him. ''For an Irish martyr? Okay. Quicker than starving yourself.''

''It is that. I can't fault you there.''

''You know what men say, in the pub? They say, name an Irish martyr. They say, go on, go on, you can't, can you?''

''I could give you a string of names,'' he said. ''They were in the paper. Two years, is that too long to remember?''

''No. But keep up, will you? The people who say this, they're Englishmen.''

''You're right. They're Englishmen,'' he said sadly. ''They can't remember bugger-all.''

Ten minutes, I thought. Ten minutes give or take. In defiance of him, I sidled up to the kitchen window. The street had fallen into its weekend torpor; the crowds were around the corner. They must be expecting her soon. There was a telephone on the kitchen worktop, right by my hand, but if I picked it up he would hear the bedroom extension give its little yip, and he would come out and kill me, not with a bullet but in some less obtrusive way that would not alert the neighbours and spoil his day.

I stood by the kettle while it boiled. I wondered: has the eye surgery been a success? When she comes out, will she be able to see as normal? Will they have to lead her? Will her eyes be bandaged?

I did not like the picture in my mind. I called out to him, to know the answer. No, he shouted back, the old eyes will be sharp as a tack.

I thought, there's not a tear in her. Not for the mother in the rain at the bus stop, or the sailor burning in the sea. She sleeps four hours a night. She lives on the fumes of whisky and the iron in the blood of her prey.

When I took back the second mug of tea, with the demerara stirred in, he had taken off his baggy sweater, which was unravelling at the cuffs; he dresses for the tomb, I thought, layer on layer but it won't keep out the cold. Under the wool he wore a faded flannel shirt. Its twisted collar curled up; I thought, he looks like a man who does his own laundry. ''Hostages to fortune?'' I said.

''No,'' he said, ''I don't get very far with the lasses.'' He passed a hand over his hair to flatten it, as if the adjustment might change his fortunes. ''No kids, well, none I know of.''

I gave him his tea. He took a gulp and winced. ''After ...'' he said.

''Yes?''

''Right after, they'll know where the shot's come from, it won't take any time for them to work that out. Once I get down the stairs and out the front door, they'll have me right there in the street. I'm going to take the gun, so as soon as they sight me they'll shoot me dead.'' He paused and then said, as if I had demurred, ''It's the best way.''

''Ah,'' I said. ''I thought you had a plan. I mean, other than getting killed.''

''What better plan could I have?'' There was only a touch of sarcasm. ''It's a godsend, this. The hospital. Your attic. Your window. You. It's cheap. It's clean. It gets the job done, and it costs one man.''

I had said to him earlier, violence solves nothing. But it was only a piety, like a grace before meat. I wasn't attending to its meaning as I said it, and if I thought about it, I felt a hypocrite. It's only what the strong preach to the weak: you never hear it the other way round; the strong don't lay down their arms. ''What if I could buy you a moment?'' I said. ''If you were to wear your jacket to the killing, and be ready to go: to leave the widowmaker here, and pick up your empty bag, and walk out like a boiler man, the way you came in?''

''As soon as I walk out of this house I'm done.''

''But if you were to walk out of the house next door?''

''And how would that be managed?'' he said. I said, ''Come with me.''

He was nervous to leave it, his sentry post, but on this promise he must. We still have five minutes, I said, and you know it, so come, leave your gun tidily under your chair. He crowded up behind me in the hall, and I had to tell him to step back so I could open the door. ''Put it on the latch,'' he advised. ''It would be a farce if we were shut out on the stairs.''

The staircases of these houses have no daylight. You can push a time-switch on the wall and flood the landings with a yellow glare. After the allotted two minutes you will be back in the dark. But the darkness is not so deep as you first think.

You stand, breathing gently, evenly, eyes adapting. Feet noiseless on the thick carpets, descend just one half-flight. Listen: the house is silent. The tenants who share this staircase are gone all day. Closed doors annul and muffle the world outside, the cackle of news bulletins from radios, the buzz of the trippers from the top of the town, even the apocalyptic roar of the aeroplanes as they dip towards Heathrow. The air, uncirculated, has a camphor smell, as if the people who first lived here were creaking open wardrobes, lifting out their mourning clothes. Neither in nor out of the house, visible but not seen, you could lurk here for an hour undisturbed, you could loiter for a day. You could sleep here; you could dream. Neither innocent nor guilty, you could skulk here for decades, while the alderman's daughter grows old: between step and step, grow old yourself, slip the noose of your name. One day Trinity Place will fall down, in a puff of plaster and powdered bone. Time will draw to a zero point, a dot: angels will pick through the ruins, kicking up the petals from the gutters, arms wrapped in tattered flags.

On the stairs, a whispered word: ''And will you kill me?'' It is a question you can only ask in the dark.

''I'll leave you gagged and taped,''he says. ''In the kitchen. You can tell them I did it the minute I burst in.''

''But when will you really do it?'' Voice a murmur. ''Just before. No time after.''

''You will not. I want to see. I'm not missing this.''

'Then I'll tie you up in the bedroom, okay? I'll tie you up with a view.''

''You could let me slip downstairs just before. I'll take a shopping bag. If nobody sees me go, I'll say I was out the whole time. But make sure to force my door, won't you? Like a break-in?''

''I see you know my job.''

''I'm learning.''

''I thought you wanted to see it happen.''

''I'd be able to hear it. It'll be like the roar from the Roman circus.''

''No. We'll not do that.'' A touch: hand brushing arm. ''Show me this thing. Whatever it is I'm here for, wasting time.''

On the half-landing there is a door. It looks like the door to a broom cupboard. But it is heavy. Heavy to pull, hand slipping on the brass knob.

''Fire door.''

He leans past and yanks it open.

Behind it, two inches away, another door.

''Push.''

He pushes. Slow glide, dark into matching dark.

The same faint, trapped, accumulating scent, the scent of the margin where the private and public worlds meet: raindrops on contract carpet, wet umbrella, damp shoe-leather, metal tang of keys, the salt of metal in palm. But this is the house next door. Look down into the dim well. It is the same, but not. You can step out of that frame and into this. A killer, you enter No 21. A plumber, you exit No 20. Beyond the fire door there are other households with other lives. Different histories lie close; they are curled like winter animals, breathing shallow, pulse undetected.

What we need, it is clear, is to buy time. A few moments' grace to deliver us from a situation that seems unnegotiable. There is a quirk in the building's structure. It is a slender chance but the only one. From the house next door he will emerge a few yards nearer the end of the street: nearer the right end, away from town and castle, away from the crime. We must assume that despite his bravado he does not intend to die if he can help it: that somewhere in the surrounding streets, illegally parked in a resident's bay or blocking a resident's drive, there is a vehicle waiting for him, to convey him beyond reach, and dissolve him as if he had never been.

He hesitates, looking into the dark.

''Try it. Do not put on the light. Do not speak. Step through.''

Who has not seen the door in the wall? It is the invalid child's consolation, the prisoner's last hope. It is the easy exit for the dying man, who perishes not in the death-grip of a rattling gasp, but passes on a sigh, like a falling feather. It is a special door and obeys no laws that govern wood or iron. No locksmith can defeat it, no bailiff kick it in; patrolling policemen pass it, because it is visible only to the eye of faith. Once through it, you return as angles and air, as sparks and flame. That the assassin was a flicker in its frame, you know. Beyond the fire door he melts, and this is how you've never seen him on the news. This is how you don't know his name, his face. This is how, to your certain knowledge, Mrs Thatcher went on living till she died. But note the door: note the wall: note the power of the door in the wall that you never saw was there. And note the cold wind that blows through it, when you open it a crack. History could always have been otherwise. For there is the time, the place, the black opportunity: the day, the hour, the slant of the light, the ice-cream van chiming from a distant road near the bypass.

And stepping back, into No 21, the assassin grunts with laughter.

''Shh!'' I say.

''Is that your great suggestion? They shoot me a bit further along the street? Okay, we'll give it a go. Exit along another line. A little surprise.''

Time is short now. We return to the bedroom. He has not said if I shall live or should make other plans. He motions me to the window. ''Open it now. Then get back.''

He is afraid of a sudden noise that might startle someone below. But though the window is heavy, and sometimes shudders in its frame, the sash slides smoothly upwards. He need not fret. The gardens are empty. But over in the hospital, beyond the fences and shrubs, there is movement. They are beginning to come out: not the official party, but a gaggle of nurses in their aprons and caps.

He takes up the widowmaker, lays her tenderly across his knees. He tips his chair forward, and because I see his hands are once more slippery with sweat I bring him a towel and he takes it without speaking, and wipes his palms. Once more I am reminded of something priestly: a sacrifice. A wasp dawdles over the sill. The scent of the gardens is watery, green. The tepid sunshine wobbles in, polishes his shabby brogues, moves shyly across the surface of the dressing table. I want to ask: when what is to happen, happens, will it be noisy? From where I sit? If I sit? Or stand? Stand where? At his shoulder? Perhaps I should kneel and pray.

And now we are seconds from the target. The terrace, the lawns, are twittering with hospital personnel. A receiving line has formed. Doctors, nurses, clerks. The chef joins it, in his whites and a toque. It is a kind of hat I have only seen in chil- dren's picture books. Despite myself, I giggle. I am conscious of every rise and fall of the assassin's breath. A hush falls: on the gardens, and on us.

High heels on the mossy path. Tippy-tap. Toddle on. She's making efforts, but getting nowhere very fast. The bag on the arm, slung like a shield. The tailored suit just as I have foreseen, the pussy-cat bow, a long loop of pearls, and – a new touch – big goggle glasses. Shading her, no doubt, from the trials of the afternoon. Hand extended, she is moving along the line. Now that we are here at last, there is all the time in the world. The gunman kneels, easing into position. He sees what I see, the glittering helmet of hair. He sees it shine like a gold coin in a gutter, he sees it big as the full moon. On the sill the wasp hovers, suspends itself in still air. One easy wink of the world's blind eye: ''Rejoice,'' he says. ''Fucking rejoice.''

• The Assassination of Margaret Thatcher is published by 4th Estate (£14.99) on 30 September. To order a copy for £11.99 with free UK p&p call Guardian book service on 0330 333 6846 or go to guardianbookshop.co.uk.