I vividly remember seeing funny-looking puppets on evening television while I was growing up and being told the ’scary lady’ was our Prime Minister Margaret Thatcher.

But little did I know the man voicing the Spitting Image caricature, who compared her ministers to vegetables and transformed into Alien, was a friendly looking, ginger-haired chap named Steve Nallon.

“I read about it in the newspapers and met producer John Lloyd,“ recalls the impressionist who was in his 20s when he was cast and worked on the ITV series for its entire 12-year run from 1984 to 1996.

“After that I met Roger Law – who did the puppet – and he loved the idea of a man playing Thatcher, more than me I think, and then I was there for the next ten years.“

They had intended to bring down the Tory government, but Steve admits that by portraying Thatcher as an indestructible monster the satirical show did just the opposite.

However, England’s only female PM was finally brought down by her once trusted advisor, Foreign Secretary Geoffrey Howe, whose speaking skills had famously been compared to those of a dead sheep.

When she sacked him in 1989, he was spurred on by wife Elspeth to overcome his limitations and destroy Mrs Thatcher with one of the great political speeches.

Now the story of what went on behind the scenes is being told for the first time in the play Dead Sheep and Steve is stepping back into the blue suit to portray Thatcher on stage for the first time.

The Barnet resident says: “The real joy of the play is we are giving an insight into their private lives. There’s a wonderful scene with Alan Clarke who is a very rude minister under Thatcher and very few of the scenes you see are public things. Some of it is very funny when you see how Mrs Thatcher fired people or got angry or stared at her ministers giving jokey references to her that she didn’t understand.“

The drama covers the period from 1980 to 1991 and marks 25 years since Howe’s speech, two years since Thatcher’s death on April 8, 2013 and will play during the General Election.

Writer Jonathan Maitland has interviewed Sir Geoffrey Howe and other Whitehall insiders to carefully construct the plot, which unfolds in a series of flashbacks.

Despite playing Thatcher more than 100 times on shows including Noel’s House Party, Newsnight and Bullseye, Steve says there have still been revelations for him.

“I hadn’t quite realised that she was a mean b**ch. I didn’t know that when Geoffrey Howe lost his job he also lost his house – because it was a grace and favour house so when she sacked him she said ’can I have the house back as well please?’ which p***** him off a bit!“

The actor, who will also star in the upcoming film 51 Degrees North, landed the part after his friend Alistair McGowan recommended him and he was asked to do a read through.

“It was bizarre – it was the middle of August and I was in shorts and trainers with my hairy legs and they seemed to quite like that,“ laughs Steve.

The 54-year-old did try to squeeze back into his old costumes but was horrified to discover they were too tight so is having a little blue number made, and complete with make-up, a wig and Thatcher’s “duck with a limp“ walk hopes he can convince the intimate audience at Park Theatre.

“It’s quite intimidating because this play is in the round. We were discussing whether or not she should wear a pair of ladies’ knickers underneath the dress because there are so many people all around you and they are so close that I have got to be careful – I could have a Sharon Stone moment where someone looks up my skirt and sees my Calvin Kleins.

“So we are going to get some frilly knickers or bloomers for her.“

Underwear flashing aside, Steve says the most challenging part for him has been finding a voice for the Thatcher we didn’t know.

“This is totally different to Spitting Image which wasn’t really how she sounded.

“For this you have to find lots of different levels. We have her in the House of Commons, we have her in private. The most difficult scene I have to do is one from early on in 1981 and she was very vulnerable.

“The fun part is the House of Commons where I get to shout – the other day in rehearsals I skipped a page by mistake but nobody dared stop me – they were too frightened to interrupt!“

The former Colindale resident, who doesn’t like any of today’s political parties, seems to have built up a grudging respect for Thatcher, quipping: “Well she bought me my house in High Barnet“, but despite never meeting her believes he knows what she would think of his portrayal.

“She would have been frosty about it – she wasn’t known for her sense of humour.“ 

Park Theatre, Compton Terrace, Finsbury Park, April 1 to May 9. Details: parktheatre.co.uk