Seven miles off the coast of Suffolk, there is a country. It isn't a very big country. In fact, its surface area extends to no more than 6,000 square feet, which is about twice the size of a tennis court. You won't find it on Google Maps and it isn't a member of Nato or, indeed, the EU. But it exists. And I know, because I've been there.

We embarked from the Thames Estuary in the dead of night. This, I was assured, was in order to catch the tide, but it gave our journey a whiff of mystery. Soon we were swathed in fog, as seagulls rose from the surface of the water, ghostly in the light cast by our 45ft fishing boat, the Charlotte Joan.

Four-and-a-half hours after we left, looming out of the darkness, there it was: the Independent Principality of Sealand. I could make out a bleak metal platform on a pair of concrete pillars, washed by the grey, unglamorous billows of the North Sea. In what sense could this possibly be said to constitute a country? Sealand has its own flag, coins, stamps. It has its own Royal Family, presided over by the majestic Prince Michael of Sealand (more commonly known as Michael Bates). And immigration isn't easy, as I was about to discover.

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There is no mooring post or landing stage (these were long ago smashed to pieces by the waves). There's no ladder. Like many small islands – like England, you might say – Sealand has suffered a painful history of invasion and counter-invasion, to which it has no particular desire to append another chapter. So the only way up on to the 60ft-high platform is by means of a swing seat, suspended from a winch.

Somehow, as our vessel listed in the waves, I managed to get my backside in place. A thumbs-up to the winch-operator, and I was hoisted into the air, my white fingers gripping the ropes on either side. Up, up, watching the Charlotte Joan diminish beneath my dangling feet; then a side-swing, and I was stood, with some relief, on relatively solid ground.

"Welcome to Sealand," said Prince James. Behind him, the national flag (red and black, with a white diagonal stripe) fluttered in the pre-dawn breeze.

This was James Bates, Michael's good-natured twentysomething son and heir apparent, who had agreed to escort me out to Sealand, along with a couple of his mates. They were performing running repairs to the place, which was why I had been preceded on the winch by a wheelbarrow and 50 bags of cement, for patching holes in the platform. As Prince Michael would put it later, my visit "worked in fine with what we were doing anyway".

But I still felt honoured, as I watched the f sun come up. After all, I was the first journalist in the history of Sealand ever to be permitted to stay overnight there.

The reason for this suspicion of strangers in general lies in the violent, picaresque nature of its past. Sealand was built in 1943 by the Royal Navy as an anti-aircraft fortress designed to shoot down Luftwaffe planes. In those days it was equipped with two 94mm Vickers heavy anti-aircraft guns and two 40mm Bofors light anti-aircraft guns, and manned by 120 seamen crammed into accommodation in the hollow concrete towers. It was known as HM Fort Roughs, or Roughs Tower for short. Abandoned after the War, it gathered rust and guano, a gloomy relic of conflict, until the era of pirate radio in the 1960s.

Then two rival entrepreneurs competed for possession, regarding the fort as the perfect place (since it was outside the three-mile zone that then constituted British territorial waters) from which to broadcast pop music to a grateful generation of teenagers. The piratical pair were the long-haired Irish chancer Ronan O'Rahilly, of Radio Caroline fame, and one Roy Bates, a cravat-wearing former Army major.

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Each time one of them put men on Roughs Tower, the other would send people to eject them, sometimes forcibly. It was a question of who was prepared to go further, and the answer turned out to be the Englishman. For Bates, the solitary fortress became far more than a radio project. It became an obsession that would absorb not only his life, but also the lives of his wife and children.

The key thing, he knew, was to maintain a presence. With even one occupant, Roughs Tower was tough to take. But Roy couldn't afford a guard, so instead he plucked his 14-year-old son Michael out of school and put him up there, sometimes with his daughter Penny, sometimes with his wife Joan. For Michael, this was a welcome escape from the dreary rigours of a public-school education, but as he confided to me during a long lunch on-shore after my visit, "I expected it to last six months, not 40-something years".

In July 1967, the Marine Broadcasting Offences Act came into force, extending the ban on unlicensed broadcasting beyond British waters. Roy dropped his proposal to set up a radio station, but the Bateses didn't leave the fort. The government, uneasy at this maverick presence a few miles off the east coast, sent a boat out there when they knew Roy was away on business.

They claimed they had his permission to come aboard, but Michael and his mum, who were holding the fort, weren't so gullible. There were even discussions in Downing Street as to whether the place should be taken by force. In his excellent, newly finished memoirs, Michael reveals: "Harold Wilson, bless him, rejected the plan, as he didn't want to see military personnel or my family injured or killed".

Uninjured, and very much alive, the family remained in situ, albeit with a precarious sense of being under siege.

"Well, darling," Roy told his wife over a drink one evening. "Now you have your very own island."

"Yes," she replied. "It's just a shame it doesn't have a few palm trees, and a bit of sunshine, and its own flag."

The last part of her not unreasonable rejoinder planted a seed in Roy's mind. Why shouldn't the place have its own flag? Why, for that matter, shouldn't it be declared a sovereign state, immune from British law? Thus was born the Independent Principality of Sealand. It wasn't just an extended Ruritanian joke (although clearly that's to some extent what it is and has always been).

It was also a snook cocked by a war veteran at a government he felt undervalued the "blood he'd spilt for his country". The legality of Roy's claim was soon put to the test. In May 1968, engineers working on a nearby buoy spotted the pretty Penny up on the fort and yelled what Michael describes as "lascivious" remarks at her. The hot-headed boy met shouts with shots, firing over their heads.

Over lunch, I asked Michael about the insults. Had they requested a date?

"No," he replied scornfully. "It was something like, 'I'd like to give you one'."

Ah. So what did you use to fire your warning shots? Was it an air rifle?

"No." A shotgun? "No." A rifle? "No." He paused. "It was an automatic pistol."

Why, Michael, did you have an automatic pistol? "To look after my sister," he laughed.

The authorities were less amused. The next time he returned to the mainland, Michael was arrested and put on trial for firearms offences. In the event, the judge threw out the case on the grounds that 'Sealand' was beyond British jurisdiction. Prince Roy, as he had taken to calling himself, hailed this as official confirmation of the place's independence. For if British law didn't apply there, in what sense was it a part of Britain? And if it wasn't British, what was it?

From out on the fort, where there is a faint phone signal, I called up the Foreign Office and requested its official position on the sovereign claims of Sealand. Its response, in part, was that it "cannot constitute a separate independent State since it has none of the characteristics of a State, such as a fixed population, a territory or the ability to conduct international relations".

We'll consider these three points in turn. Regarding the first, there has never been a time since the 1960s when there wasn't at least one person on Sealand. From 1966 until he got married in 1986, it was mostly Michael. Roy himself was there for the 1990s. Since then, there has been a succession of caretakers who alternate, shuttling between Sealand and the mainland every few weeks.

During my visit, the guard in question was Mike: a stout ex-engineer who, when he wasn't telling me to fuck off, or worse, proved remarkably knowledgeable about the country he ruled as regent. I was beginning to see that, depending on your point of view, Sealand was a museum of wartime history, a kid's dream (a kind of über-den in the North Sea), or a skanky rust-bucket.

I inclined to the first two of these perceptions, although I was glad my girlfriend wasn't with me. For one thing, the place is freezing. When Joan Bates was in residence in the 1960s, she used to faint from the cold while hanging out the washing on deck. The accommodation is basic. I slept on a sofa, though my room was blessed, unlike others, with a free-standing Calor gas heater. The bunks that would have housed the wartime inhabitants have long since gone, leaving only holes where they were screwed into the curved concrete walls.

While giving me a tour of the two towers, Mike pointed out pockmarks in the side of a cupboard, clustered around a pristine circle, where a dartboard once hung. Except when manning the guns, the seamen weren't allowed to leave their claustrophobic quarters, but were stuck inside – underwater, in the case of the lower floors – their senses lulled by the repetitive slosh and gurgle of the sea. Yet even for those up on deck, the sense of isolation would have been acute. There's nothing to see but sea; the odd buoy; and in the west, the grey outline of England.

"I actually like it," Mike said, when I asked him about the solitude. "I love it." Then his face darkened, and he lowered his voice. "All this [building work] at the moment is getting me down. I'm sorry I took it out on you, but I get bloody fucked off sometimes. But I am an awkward person to live with and I know that." I knew it, too. There had been an incident over breakfast that day, and another the morning before. I was learning to handle Mike.

I admired him, I suppose, not only for his knowledge of Sealand and of pirate radio generally (he worked in the 1980s on one of the Radio Caroline ships) but also for his self-sufficiency. This is a man who, when at his on-shore home, has a sign on his front door: CALLERS NOT WELCOME. Vaguely religious, he has adapted one of the rooms in the fort into a chapel. There, beside an open Bible, I spotted a copy of the Koran, and works of Plato and Shakespeare. I remarked to Mike that he must have a lot of time for reading. Not his thing, he replied, although he enjoyed poring over the generator manual.

The inhabitants of Sealand haven't always been content. Michael's sister Penny soon decided she'd had enough, and since then has had nothing to do with the place. And Michael himself has decidedly mixed feelings about the nearly two decades he spent there. There was no internet, then. No mobile phones. And more importantly, for a lad growing up, no women. Michael used to chat up girls on Sealand's radio. He even persuaded one of them to come out and visit him at the weekends, cadging a lift from a local fisherman.

"We had a whale of a time," he told me – or at least, they did until his old man found out and read him the riot act. Roy Snr was for some reason convinced The News of the World were going to splash it over their front page. He was paranoid about the press. ("Never trust a journalist," he used to say.) Michael is more relaxed, which may partly explain why, following the passing of his father last year, he eventually agreed to my persistent requests to be allowed to stay on Sealand. I noticed, however, that he hadn't chosen to come, too.

So there were five of us: Mike, myself, James Bates and his two mates. It was supposed to be a long weekend but it grew longer after the weather turned on Sunday. The sea and sky got intimate. Great waves crashed around the towers with a sound like the boom of cannon. The FCO claims Sealand has no territory, but if that refers to any fixed structure, I can confirm that, much to my relief, it qualifies. The waves crashed but the fortress didn't shake. Nevertheless, there was no question, in such turbulent conditions, of anyone coming out to pick us up. We were trapped on Sealand. Worse than that, we'd run out of booze.

I had brought out a couple of big bottles of whiskey with me as peace offerings – I had guessed Mike might prove difficult – but we'd drunk them by the second day. My only set of clothes was getting fruity. Michael, meanwhile, was tucked up at his home on the mainland.

Since his marriage, the second Prince of Sealand hasn't lived out there, devoting his energies to running a fishing business from Leigh-on-Sea, Essex. Yet if anyone has done his time on the fort, he has. And it wasn't all roses. There was the incident in 1978, when his father was in discussions with a shady group of Dutch and German businessmen who wanted to turn Sealand into a casino.

They lured Roy to a meeting in Austria, and meanwhile, sent a helicopter to the fort. The sole occupant, Michael, was suspicious. But he let them land, only to find himself taken prisoner for several days. After his release, he was reunited with his father, by then furious that his son had let the interlopers outwit him. His mother was just as harsh. "You've thrown away our life's work," she hissed. Once everyone had calmed down, they discussed what should be done.

"We were going to go out in an inflatable, and scale [Sealand] using ladders," Michael told me, in his characteristically matter-of-fact tone. Instead, the Bateses persuaded a friend who had worked as a stunt pilot in a couple of James Bond films to fly them and two mates out in his helicopter. They arrived at dawn, approaching into the wind so the bad guys wouldn't hear the hum of the rotors.

Michael had a sawn-off shotgun. The others had pistols. They couldn't land the helicopter, so the pilot held the machine in a hover 40 foot up, while Michael and his dad shinned down ropes they had tied to the seats. The enemy emerged to see what the noise was. As Michael hit the deck, the butt of his gun smacked against it, and the thing fired, almost blowing his head off. Terrified, the Dutch and German heavies surrendered. The battle was over.

The unwanted guests were sent packing, with the exception of a German lawyer named Gernot Putz. As the weeks went by, the German authorities became concerned about Putz's fate. Finally, they sent out a diplomat from their embassy in London to sue for his release. Which brings us to the third FCO observation: that Sealand lacks "the ability to conduct international relations". Clearly this wasn't the case back in 1978.

Why, you may wonder, did Roy (and why does Michael) care so much about the independence issue? It's because the claim was, and is, crucial to the place's commercial prospects. On the one hand, there's the merchandise, the mugs and T-shirts, which are now sold online. There are the noble Sealand titles, which the country began bestowing on anyone who did it service. Which means, as often as not, providing funds.

The place now pays for itself but it's a delicate balance, which depends on the viability, however moot, of its sovereignty. This is why the UK government's decision in 1987 to extend its territorial waters from three to 12 miles was potentially disastrous. With typical élan, Roy Bates pre-empted the move by declaring, the day before the Territorial Sea Act was passed, that Sealand had also decided to extend its waters from three miles to 12. As a result, he said, the Suffolk port of Felixstowe now belonged to him.

In the statement it sent me, the FCO continued: "Upon the coming into force of the Territorial Sea Act, Roughs Tower thereafter fell within the territorial sea of the United Kingdom, and is therefore subject to our jurisdiction. It follows that, for example, English criminal law would apply there."

Well, it might in theory, but it doesn't in practice, it seems. There was another incident in 1990 when warning shots were fired from the fort over a boat that came too close, but no charges were brought. International law is a complex, partly theoretical affair, and I'm no expert. However, I spoke to someone who is: he told me that in these cases the law comes down to a matter of what can be enforced. Clearly, if it wanted to, the British government could take the fort from the Bateses without breaking a sweat. Mike the guard told me that, when the tide is full and the sea calm, he stays up all night, watching for a marine assault.

There was no danger of that, while I was there. Yet by my fifth day on Sealand, the waters settled enough for a boat to reach us. As I left, Mike grasped my hand with surprising warmth. He was happy to see me – but happier to see me go.

Back in Leigh, absorbing a pint in Ye Olde Smack, I asked Prince Michael what the future held for Sealand.

"It's great that I have my sons involved," he confessed, "It's so much easier."

There's a film script in the works, which has been snapped up by Hollywood's CAA, with Liam Neeson mooted for the lead. There are Michael's memoirs. There's the Sealand international football team, comprised of honorary citizens such as the actor Ralf Little. Their opponents so far have included the Channel island of Alderney and the Chagos Islands in the Indian Ocean.

And there's a plan about to be put into action for a data storage company called HavenCo to return to the fort. They came in 1999 during the dotcom boom, but left when that bubble burst a few years later. Now they're set to try again, the idea being to provide a secure facility where companies, or even countries, can store sensitive information. And what could be more secure than a sea-fortress which isn't subject to conventional regulations? Yet here, as ever, Sealand has to be careful. It will draw the line at flagrant illegality.

As Michael put it to me, "If you want to be treated like a state, you have to act like one".

Which takes us back to the Realpolitik theory of international law. If the cyber-geeks were to go too far and prompt the British authorities to move in, the likely result would be not only the closing-down of HavenCo, but also the end of Sealand's experiment in nationhood. It must be said, though, that the Bateses' personal fiefdom has negotiated nearly five decades with impunity, while convincingly dispensing with less robust threats. There's no reason to think it will blow it now.