It really was a time warp, topped by a ghost. There we were, sitting in the Curve, Leicester, an ultra-modern theatre, where the side walls of the stage can be lifted into the air, so that passers-by can see what goes on outside performance hours. But on the stage itself, we were being whisked back 40 years to an art deco former cinema, hidden deep underground in Lower Regent Street, London. For it was there, in the BBC’s Paris Studio, that the famed radio show Round the Horne was recorded, along with its successor Stop Messing About.

And the ghost? He was Kenneth Williams, for whom Stop Messing About was created, following the sudden death of Kenneth Horne in 1969. In the stage version of Stop Messing About, Williams is played by Robin Sebastian (pictured right), and when he first appears, it’s scary: the trademark Williams mannerisms and camped-up voices are so uncannily accurate.

“I’ve done impersonations before, but not to this extent,” Sebastian told me after the performance. “I listen to Round the Horne and Stop Messing About ad nauseam – it’s all on my phone actually; there’s always stuff I haven’t listened to, there is so much material out there. When we first started doing this show, it was fun trying to find out where the original recordings of various sketches were.”

As he talked, Sebastian effortlessly switched backwards and forwards from his own to Williams’s style of delivery.

“Williams takes me over,” he laughed as he briefly switched to a received pronounciation accent. “I was looking at him in Carry On – Don’t Lose Your Head over the weekend, and I was just watching how all the different muscle groups worked in his face. Then there’s the oscillation of the different sounds he comes up with, from being posh to being [and here Sebastian’s voice became pure Williams] ‘as common as muck, you know’. I love it, I really do.”

“My hillocks have been held by each army in turn,” cries India Fisher, playing Stop Messing About regular Joan Sims, in a send-up of Gone With the Wind. Meanwhile Williams relishes the line: “It’s every southern boy’s duty to go up there and get himself a Yank”. The assorted sketches that make up the stage show have been assembled by one of the original radio show writers, Brian Cooke.

“I’ve still got every script I ever wrote: hundreds of them, thousands even,” Cooke revealed. “I went through them all. It was actually great fun to work with that material from all those years ago. It’s astonishing how little things have changed. I guess a joke’s a joke: if it was funny then, it’s still funny now. Mind you, I’ve cut out those that weren’t funny! As for the double-entendres, I think you can get away with more in the theatre than you could on radio then – although we never had any problem with John Simmonds, our producer. He would defend us writers to the death.”

The stage set features four microphones, with each actor stepping smartly up to one of them when it’s their turn to deliver a line. Nigel Harrison, playing Hugh Paddick, changes mic each time he switches into a different character, and voice. But all that is to add visual interest for today’s theatre audiences – originally it was much more basic, Brian Cooke explained. “There was only one microphone, and they were all grouped around it. A show would all be done in one day. They’d come in, often not having read the script beforehand. They’d read the script through a couple of times, to discover where the jokes were – sometimes they were quite hidden. Then they would go out and do the show, in front of an audience, with songs and a band.

“There was very little retaking. You were only booked in to the Paris for a limited time. One of the things you have to remember is that weren’t any canned laughter machines then – so if you did retake something afterwards, you’d have to find some way of splicing the laughter back in.”

“What I find amazing is that we do the show again and again, and if something isn’t working, we can change it,” added Robin Sebastian. “To be able to make the right choices just like that, on virtually no rehearsal, is incredible. It shows what an absolute genius Kenneth Williams was. It’s such a damned shame he’s gone, well before his time.”

Stop Messing About is at the New Theatre, Oxford, April 6-8 (tickets: 0844 847 1585) and at Royal & Derngate, Northampton, April 15-17 (tickets: 01604 624811).