Their toothy grins and catchy tunes made them the first family of pop.

Dressed in matching jumpsuits and with smiles that set adolescent hearts fluttering, The Osmonds were the squeaky-clean heart-throbs of a generation.

Almost 40 years on, the hair is thinner and the waistlines thicker, but thousands of their fans still wear the band’s pictures on their T-shirts and follow them around the world waving their hands when little Jimmy sings Long Haired Lover From Liverpool.

The youngest of the seven singing siblings, Jimmy is about to hit the stage at the Birmingham Hippodrome as smooth-talking lawyer Billy Flynn in musical Chicago, from June 1-13.

The role, which has been played by everyone from RnB singer Usher to Baywatch star David Hasselhoff, is a world away from his better-known guise as the squeaky-voiced youngster wearing a Christmas jumper on Top of the Pops. But he insists it is not so far from the real him – the businessman Jimmy.

“If you knew what I was like outside of work you would realise he is very like me,” he says. “I manage people, I get stuff done, and I like to think I can do it with a little sugar.”

At their height in the 1970s, teenage girls would do almost anything to get up close to their heores.

As Jimmy sits with wife Michelle in a back room of the Birmingham Hippodrome, it is a quieter affair – and he had to be smuggled in quietly so no-one knows he’s there.

“It always amazes me what people do and I always appreciate it,” says Jimmy, now a 46-year-old father of four. “Two girls once mailed themselves to us. They had shut themselves in this box and got it mailed when we were at an arena somewhere in England. They had been in there about two days when they arrived and it was pretty nasty. They were in there and there was all this food wrapping.

“There was another time at the real height of it in the 70s when a porter at a hotel we were staying at got threatened with a knife because he wouldn’t let them upstairs. We all had to be padlocked inside for our own safety. Another time these ropes appeared over a hotel balcony and there were two girls trying to hoist themselves down to us.”

Last year the brothers performed their farewell tour. The eldest of the group, Alan, is suffering from MS and has officially retired from showbusiness, but they still team up in trios or quartets to plays shows featuring all of their classics.

Jimmy adds: “I don’t take myself very seriously. If I didn’t perform some of our songs again I don’t think it would be a bad thing, but I don’t regret any of it, they were good times.

“I hate it when bands come on stage and start playing all this new music, it’s not what people want to hear. We were playing at Wembley and I left Long Haired Lover out of the set. They all started shouting for it, stamping their feet, and I realised, it’s not about me, it’s about the audience and their memories.”