If you ever find yourself moving in showbiz circles and manage to get up to something embarrassing or slightly illegal, it would be best to avoid confiding in one particular person – that indiscreet diarist to the stars, actor Richard E Grant.

The Withnail & I star has one trait which means he never forgets any of his cohorts’ sozzled misdemeanours: He doesn’t drink.

His abstinence means that he can stand back and watch whatever transpires, safe in the sober knowledge that he’ll have total recall when it comes to jotting down his daily memoirs in the long gaps between filming. Diary writing has been one of Grant’s preoccupations since he was a child, but the opportunities afforded by these long, boring bouts of enforced idleness on location gave them an added spiciness.

A perfect example of this happened in the Czech Republic, where 41-year-old Grant was filming his lead role in the BBC1 costume fopathon The Scarlet Pimpernel, where he’ll be seen as Sir Percy Blakeney, the scourge of the French Revolution’s Madame Guillotine, and the splendid chap who saved many a Parisian powder-puff from her steely condemnation.

“I write about everything and everybody,” he says with a wicked twinkle. “I managed quite a lot while in Prague. There was only one place to go for any night life and everybody went there all the time. They served absinthe, which is illegal in most of the rest of the world and can blow your head off. “People had no memory of what they said or did. But, as I don’t drink, I could help them totter out – and I remembered everything!”

His own memories of the sojourn in eastern Europe are ones for which this most thespy of actors feels little affection. Even though he was brought up in Swaziland and retains dual UK nationality with the African state, he was extra glad to be back in Blighty.

“After five months in Prague, I was like the Pope,” he confesses. “I wanted to kiss the concrete in front of Marks and Spencer’s and Sainsbury’s. It was so wonderful to hear English again and have something other than wild boar and dumplings on the menu.”

There were plus points to the trip, though. “It was worth it,” he concedes grudgingly. “All the locations look like 18th century French engravings and you could get 1,000 extras in the crowd scenes for the price of 100 back in England.” The new series, though, is no bargain basement three-parter. Against these lavish backdrops, wearing sumptuous costumes and swashbuckling like a pro, Grant co-stars with American actress Elizabeth McGovern, who plays Lady Marguerite Blakeney, and Martin Shaw as the villainous Chauvelin.

As you might expect, Grant is sensitive enough to realise that there could well be republican mutterings about the dramatisation of The Scarlet Pimpernel, someone who saved so many French aristocrats from their imminent decapitation.

“I can hear it now,” he grimaces. “‘How dare the BBC spend money promoting upper-class twittage?’ But this is focused on the idea of saving people’s lives – and not all aristocrats were heartless slave-beaters.

“I’m not a Royalist by any means, but I have no desire to have the Royal family beheaded and stuck on posts down The Mall.”

There can be few English actors who could have provided the depth necessary for playing such a flamboyant part and, indeed, the Pimpernel’s action-packed derring-do is another trait shared by the man chosen to play him.

“He was an adrenaline junkie,” says Grant. “He could live the good life, but chose to throw himself into life-risking danger. “I can understand that – needing something that gives you a big thrill. A couple of years ago, before my 40th birthday, I did a bungee jump in France. It was absolutely, staggeringly terrifying – and I would recommend it.”

Grant took his life in his hands in a rather different way more than 15 years ago, when he left that comfortable life in Swaziland, where his father was the director of education, and came to Britain to try his luck as an actor. He spent two years doing odd jobs, waiting on tables, driving and facing rejection at endless auditions. Then came Withnail, in which he co-starred with Paul McGann, as a couple of bargain basement hedonists hooked on sloth and meths in equal quantities.

Yet it was only two years ago that he found his true niche – according to his 10-year-old daughter Olivia, anyway.

“I was in SpiceWorld: The Movie,” he explains. “Everyone told me not to do it – even my agent. But it has made me a star to little girls everywhere!”

Grant’s colourful turn of phrase has won him a whole new career as a writer, first with the publication of his diaries, then with his recently published first novel By Design, a surreal tale of Hollywood types.

“I only cashed the advance cheque after I’d written half of it,” he grins sheepishly. “Staring at that blank computer screen, I was convinced I would never even start it. “I was told only to ‘write what you know’, and believe me, everything in the book is absolutely true. If you think you recognise some of the characters, you’re right!”