They were one of those bands where the definite article was optional. Mud and Slade dispensed with the "the"; the Strawbs and the Glitter Band would have been naked without one. But was it Sweet or the Sweet? It wasn't a burning question in 1973, or not as burning as "does anyone know the way to blockbuster?". Only now is it an issue. They split up years ago, but glam rock nostalgia being the honey pot that it is, there are two rival Sweets out on the road, one fronted by guitarist Andy Scott, the other by singer Brian Connolly. Remember "Ballroom Blitz"? Over the drummed intro Connolly used to ask Steve, Mick and Andy if they were ready to go. These days it's Glen, Dave, Geoff and (a different) Steve. Which Sweet is the genuine article? Which band is the Sweet?

All this would be academic if the band would only take up one of two offers tabled this year to reform. But Andy and Brian don't get on, apparently never did. "Andy didn't really come back on the scene until about five years ago," says Connolly. "Little by little he stabbed me in the back and he's taken on a lot of my work. He started ringing up promoters and my name got blackened for no reason at all." Yet the Eagles were on a civil war footing and never let it get in the way of a money-spinning reunion. The problem, it seems, lies with Steve Priest, the tub of lard on bass who looked most at home in the capes and eye liner that the band donned at their peak. He lives in paunchy retirement in LA, from which vantage point the road doubtless looks less alluring. "He didn't want to do live performances," reckons Connolly. "Having seen him recently I can see why."

You have to see Connolly in the flesh to appreciate the full glory of this statement. He was the one member of the band who didn't look like a tree trunk smeared in lipstick. He still wears his white-blond hair in a shoulder-length feather cut, but the grimacing face it frames has raced ahead. His midriff is oddly distended, he walks with gingerly grandmother's footsteps, as if still teetering on platforms, and the hands have a very pronounced shake. When he picks up a full cup of tea you fear the worst. He recently spent his 49th birthday in bed with flu. When last year Channel 4 spent an evening gleefully remembering glam rock, there was a long shadow cast by Connolly, the addled ghost of a pin-up. Very much a case of recognise your age, it's the old age rampage.

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What happened? Channel 4 has gone back to Connolly to find out, and the story is the perfect fit for the Fame Factor season, which examines the dark undertow of celebrity. The rest of the band insists that Connolly didn't have the constitution for excess, but that he indulged as much as the rest of them anyway. Connolly insists that he "didn't drink and I've never been into drugs". But when he left the band in 1979, exhausted by the lion's share of promotional chores and intent on chasing up other offers, a mole within put it about that he was an alcoholic. "I couldn't battle against what was already printed. But I did a stupid thing, because about a year after I left the group I started drinking. I was just thinking, `What they're going to say about me if they see me they've already said'."

In 1981 he had a heart attack, then another 13 in the space of 24 hours if you believe what his ex-wife says in the film, in six weeks if you believe Connolly. "I know I did the impossible but that is impossible." Whatever, you get the impression that his daughter Nicola, born at the height of Sweet's fame, commutes to his home down a mock-Tudor cul-de- sac in the Middlesex green belt as much to look after her father as his one-year-old son BJ (Brian junior). Halfway through the interview she pops out to buy his lottery ticket and some toothpaste. ("What's the one I normally use?" he says. "Colgate?" "I'll just get you a toothpaste for smokers.")

He may be lucid and surprisingly likeable, but like other pop stars with a memory laundered by the sins of success, Connolly is an unreliable witness in his own life story. Who wouldn't be shaky on the details after receiving the last rites, a prelude to two years of elocution lessons and breathing therapy? "I lost my speech completely. I had to start from scratch. Prior to the cardiacs I had a Scottish accent."

His biography as he tells it begins at the age of 12 with a move from Glasgow to Middlesex. He joined the navy at 16. "I've always been mad on boats." When he left at 18 he found out that he had been adopted at birth, and promptly dropped his foster name, McManus. (This year, he made contact with blood relatives for the first time.)

He studied "some sort of technical engineering but I was hopeless at draught work" and music was a distraction. "I was in the original band that was called Generation X. We used to spend all our time rehearsing at college. I missed so many lectures I got the push."

He fetched up in another band called Wainwright's Gentlemen, with Steve Priest and Mick Tucker. They hung around with members of Deep Purple (Connolly was a replacement for Ian Gillan, and lodged with Roger Glover in Uxbridge), and were instinctively just as keen on decibels as Purple, "not heavy but West Coast, more solid than what was the simple pop then". Somehow, though, as Wainwright's Gentlemen mutated into Sweet and Connolly gave up his job in a carpet shop, they ended up sounding as saccharine as their new name.

"There were a lot of bands around called sort of sugary names. There was Strawberry Jam, Marmalade, Orange Bicycle, Tangerine Peel, Clockwork Orange, after the film. So we thought, to sum it all up, we used a name that instigates all of them." But in those days bands went where the material was, and the songwriting pair who supplied them, millionaire's son Nicky Chinn and Australian waiter Mike Chapman, traded in bubblegum. The early hits ("Funny Funny", "Co-Co", "Chop Chop" ) were formulaically twee. "Little Willy", "Wig Wam Bam" and "Alexander Graham Bell", the only hit anyone has ever had singing about the inventor of the telephone, found the Sweet sound struggling to break free of its chrysalis, to emerge fully formed in the band's four most expressive statements of high-octane nonsense: "Blockbuster!", "Hell Raiser", "Ballroom Blitz" and "Teenage Rampage".

"Blockbuster!", their only number one, had exactly the same opening riff as "The Jean Genie", and posterity has always jumped to the conclusion that Bowie got there first. "When we took the acetate into the RCA promotion department for the first time to let them hear what their next single was going to be, Bowie was there. I remember him saying, `That's a great song, that's definitely a winner'. The first play `The Jean Genie' got," recalls Connolly, "was on Luxembourg about one or two o'clock in the morning and we were on our way back from a gig and we were all sort of half asleep in the car, the radio was on and we heard the intro and we thought somebody had lifted the siren off and nicked our back-track."

Sweet had five more years on the gravy train, but Chinn and Chapman were diversifying into Suzi Quatro and Mud - one week they had songs sung by the three acts at one, two and three in the charts. Sweet, always a singles outfit, had a go at writing their own hits: they managed a couple in "Fox on the Run" and, three years and several tours of the US later, "Love is Like Oxygen". By the time the new Generation X and their like brought the curtain down on the feather-cut, they had sold 50 million records.

The money still trickles in, some from back payments and much of it from Germany, "still probably our best market". "I could live without working," says Connolly, "but I couldn't live without working, if you see what I mean." But not, it seems, as part of the Sweet. As Andy Scott, the band's chief wordsmith, makes crystal clear on the Channel 4 programme, "It's like an egg. It's been fried. You can't unfry it, so best to just let it run its course and we'll see what comes out at the end."

`Don't Leave Me This Way' is on Channel 4 on 2 Nov