Earlier this summer, while England were licking their World Cup wounds and Cheryl Cole was recovering from malaria, the funniest TV adverts of 2010 appeared on our screens. They starred Andy Scott, the guitarist from the 1970s glam rock band Sweet, in the unlikely role of an expert on van insurance. The winning mix of nostalgia and self-parody (after explaining how easily a van insurance deal can be found, Scott exclaims "Sweet!") proved a big hit with viewers, and a follow-up ad was commissioned – it will air next month.

Scott, 61, joins an illustrious band of unlikely rock stars who have made the leap to advertising, including Iggy Pop, John Lydon and Alice Cooper. But he admits luck was involved: the owner of the insurance company just happens to be a friend and a lifelong Sweet fan. Scott still tours with a line-up of Sweet today, but you're unlikely to see them on television. Gone are the days of Block Buster! and The Ballroom Blitz, when Scott appeared on Top of the Pops more often than some of the presenters.

"I'm a realist," says Scott, nowadays a Wiltshire-based family man. "I can't survive on fresh air, so I have to work. As long as other bands keep going, why shouldn't Sweet? There's still an audience for our music in Germany. We've played in front of castles, and in stadthalles [indoor arenas]. We've done the 70s package tours in England, with Showaddywaddy and the Rubettes. We go on in the middle. Unfortunately, we can't play all the hits in the time they give us, but the gigs are fun and they pay quite well."

Scott was 20 when he joined Sweet in 1970, after moving to London from his native north Wales. A skilled rock guitarist – his scorching solo on Sweet's 1973 B-side Burning would do credit to Jimmy Page or Jeff Beck – Scott, like his bandmates, acceded to the strict rules of bubblegum pop on Sweet's early hit singles Funny Funny, Co-Co and Poppa Joe. It was either that or look for another job. The songs were written by the prolific Chinnichap partnership (Nicky Chinn and Mike Chapman), and recorded by session musicians – Sweet merely sang on them. After their fourth hit, Little Willy, Chapman realised Sweet were better than the session musos, and a compromise was reached. The band could play on the A-sides (Wig-Wam Bam, Block Buster!, Hell Raiser), which Chinnichap would continue to write, and the B-sides would showcase Sweet's songwriting and Scott's hard-rock riffs.

Sweet rose to the challenge set by Chapman's songs with ever more outlandish TV performances. Their golden-haired singer, Brian Connolly, snapped microphone stands over his knee as Scott blew him kisses. Bassist Steve Priest sported eyeliner, fake lashes and anything from red hot pants to a Nazi officer's uniform. None of the band was gay, in fact, and Priest was actually a quiet man who disliked the limelight.

"Glam rock started with something as stupid as Marc Bolan wearing a pink boa," Priest tells me from his California home. "The gates opened and we all poured through. Eventually, Bolan, Bowie and Sweet were all outdoing each other. It's amazing how everyone still talks about the Nazi uniform. Good old BBC wardrobe department. People always want to know if I was serious. I mean, a gay Hitler. Hello?!"

Musically, Sweet shared the same aspirations as Deep Purple – gigs in city halls to denim-clad audiences – but found themselves playing Top Rank and Mecca ballrooms where hysterical girls would pull them from the stage and hack wildly at their hair with scissors smuggled in their handbags. At one show in Kilmarnock in early 1973, with Block Buster! at No 1 in the charts, Sweet received trouble from both sexes. The men spat at them from balconies above the stage, and the women screamed so loudly they drowned out the music. In Liverpool, the band's driver panicked when fans climbed on the roof of the limousine, and accelerated down a nearby alley which, Priest calculates, "was probably an inch wider than the car. To this day, I have no idea how it was possible."

To relax, Sweet liked to go out on the town in a 16-strong group with their road crew, dressed in leather jackets, looking for action. "If it breathed and was female, it was fair game," Priest says unrepentantly. "The 70s were magical. They were like the 60s, only crazier. God knows how we got away with it." A friendly rivalry was born with Slade. "Lovely blokes," he remembers. "When we did Top of the Pops together, I'd spend more time in their dressing room than ours."

But in early 1974, in the midst of session for a new album that was to showcase a harder rock direction, came the incident that changed everything. Connolly was beaten up outside a pub in Staines, Surrey. Scott says Connolly was trying to protect his Mercedes from a couple of local vandals. Priest's version is much more sinister. Connolly's car was tailed by persons unknown who waited until he stopped at the pub to buy cigarettes. "It was a set-up job," Priest says. "He'd annoyed someone. There were three guys attacking him and one of them kicked him in the throat. Brian heard him say, 'That should do the job.' The only one who knows the truth is an ex-roadie of ours, and he won't tell."

The assault on Connolly changed the destiny of Sweet. As well as damaging his vocal cords, it shattered his confidence and he began drinking heavily. It also meant cancelling the most important concert of Sweet's career. Pete Townshend had invited them to support the Who at Charlton Athletic's football ground in May 1974, where Scott believes they would have proved to critics and fans alike that they were serious rock contenders, not a superficial glam machine.

Connolly's deterioration was too subtle for TV's pop audiences to notice at first. As a young Sweet fan, I thought he sang terrifically on Fox on the Run and Action in 1975. But his alcoholism put him on a collision course with Scott, who was trying to steer the band through the choppy waters of the mid-to-late 70s. As punk, new wave and disco arrived, Sweet suddenly looked old-fashioned. Connolly and Sweet parted company in 1979. In 1981, the year the remaining three decided to disband, Connolly was rushed to hospital, where he suffered 14 cardiac arrests in 24 hours. He was left with slurred speech, partial paralysis and violent tremors.

In 1988, out of the blue, Mike Chapman contacted them, offering to reunite the classic lineup and finance a recording session in Los Angeles. He was in for a ghastly surprise. "I met them at the airport and Andy and Mick came off the plane," Chapman recalls. "I said, 'Where's Brian?' They said, 'Oh, he's coming.' All the people had come off the plane by now. Then this little old man hobbled towards us. He was shaking, and had a ghostly white face. I thought, 'Oh, Jesus Christ.' It was horrifying." It became clear in the studio that Connolly's voice lay in ruins. Priest remembers Chapman taking him aside and saying, "This is like icing a cake with shit." Sweet's reunion was aborted.

Connolly died in 1997. He had apparently been very distressed by a Channel 4 documentary that had followed his desperate, hopeless attempts to join the glam rock comeback trail. Scott calls the programme "sickening". Heartbreaking might be a better word. But there was more sadness to come. In 2002 ,Mick Tucker, an underrated drummer who had been forgotten by all but the most loyal Sweet fans, succumbed to leukaemia. Then, in September 2009, Scott was diagnosed with prostate cancer. Earlier this year he underwent a course of radiotherapy treatment to save his life. Choosing his words carefully, he describes himself as "doing well, in remission, on a cocktail of half a dozen pills a day, but not cured". Priest, now 62, lives in La Cañada Flintridge, a high-income haven just outside Los Angeles, and tells me his health is fine.

Priest, like Scott, is a grandfather. Like Scott, he describes his finances as "comfortable". Like Scott, he's bewildered by a music business that has changed out of all recognition since Sweet's heyday. But the comparisons between Priest and Scott end there. The two men have not met in years and refuse to have anything to do with each other. "I have a damn good band out here," says Priest, who is allowed to tour under Sweet's name in America and Canada. "I've got musicians who've come up through the LA metal scene. We play to rock audiences, kids of 15, people of 50, big bikers. I would never do a 70s revival gig in a million years. Where's Andy been playing? Butlins?"

The two Sweets stay out of each other's territories. Livelihoods are at stake, and if a promoter is uncertain which lineup of a band to book, he ends up booking neither. Scott has faced a challenge from rival Sweets before – Connolly fronted a few in the 80s and 90s – and is confident Priest will not encroach on his trademark in Britain or mainland Europe. But there are grey areas, and one of them is Australia. In 2009, Scott's lineup cancelled some dates on an Australian tour. Priest, seeing an opening in a market where he had not played for decades, emailed the promoter, offering to fly over and take Scott's place. It was a dreadful faux pas. Scott had just been diagnosed with cancer, and regarded Priest's act of opportunism as reprehensible. For his part, Priest protests that he had no idea Scott was so ill.

If only for sentimental reasons, I tell Priest that I wish he'd pick up the phone and send Scott his regards. You should never meet your childhood heroes, they say, but I'm secretly delighted when both Scott and Priest suggest "going for a pint" next time they're in my neck of the woods. Perhaps I could engineer a sneaky reconciliation – Priest entering the pub cautiously, in full makeup and Nazi officer's uniform, to be greeted by Scott exclaiming "Sweet!" Or perhaps I should let sleeping dogs like. "It's up to him," Priest tells me, none too enthusiastically. Scott counters, "He's obviously got a problem with me. What the hell could be of such consequence? I thought I knew the man. I obviously didn't."

Priest finally gives me some hope. "I'm pretty easygoing. I don't hold a grudge. I'm not averse to writing some songs with Andy again one day."

The next time I see Scott advertising van insurance, forgive me for hoping that day comes before it's too late.