ANY minion worth his salt knows not to disturb a thesp retired to his dressing room to consult with his muse, or more likely one of the chorus girls.

But pressing your ear to Ben Nealon’s door you won’t catch the tell-tale shuffle and scuffle of such shenanigans, only strange talk about water pumps, Malawi and sanitation.

Since the launch of his relief charity Pump Aid 17 years ago, the actor has taken to using changing rooms and cramped cupboards across the nation’s theatres as his makeshift office.

“It was very hands on at first,” he says with a chuckle. “We built it up from nothing and it did take over my life at times. I would try to run the organisation out of my dressing room, having meetings on Skype. There was no point doing it if I was not going to get deeply involved.”

The 48-year-old was filming scenes for Soldier Soldier, his breakthrough TV series, in Zimbabwe, when he befriended a local teacher hired as an extra. The man was building a water pump with fellow teachers and asked for Ben’s help to set up a charity.

The actor hopped on board and Pump Aid was born.

“Funnily enough I had suggested a story line to the writer for my character: I would leave the army and join a charity. Art imitated reality or reality imitated art,” he laughs.

“That’s what happened when we created Pump Aid. I wanted to set up something that would make a difference, something that was completely sustainable. In the early stages I would build the pumps. There was no point talking about it if I didn’t know how it worked.”

Nearly 20 years on, the charity now provides clean water to more than a million people on the African continent and Ben has been made an OBE.

Pump Aid has not been the only constant in his life. Over the past 10 years, his love affair with charity work has only been matched in intensity by his dalliance with Agatha Christie’s murder mysteries.

After seven successful runs of the wordsmith’s grittiest plays with The Agatha Christie Theatre Company, he is back on tour, this time as Captain Philip Lombard in And Then There Were None, her most sinuous plot to date.

“Agatha Christie is fantastic,” he enthuses. “To see a play first done in 1944 still get that same reaction from the audience in 2015 is great. It does what it says on the tin. It’s a thriller and it’s gruesome but also funny at times. The audience screams, ooohhs and aaahhs trying to work out who the killer is. It’s great fun.”

Adapted by Christie herself from her best-selling novel, the play follows 10 strangers each lured to a remote island off the coast of Devon. They soon discover that their host, an eccentric millionaire, is missing. At dinner a recorded message is played accusing each of them in turn of harbouring a dark secret. Stranded on the island by a torrential storm and haunted by an ancient nursery rhyme, one by one the guests are brutally murdered.

Any hopes of wangling any juicy spoilers are promptly crushed. Ben has been sworn to secrecy.

“I make it to the end of the play,” he caves. “I’m there on stage but whether I’m dead or alive, people will have to see.”

The inscrutable Lombard is one of the most elusive and exciting of Christie’s characters he has played to date.

“You’re never quite sure what he’s about. On the face of it he’s pretty nasty but there is this charm to him. You keep thinking he has to be the killer and then, ‘No, he can’t be’. Of all the characters that I’ve played in the last 10 years he is the best so far.”

Wise beyond his years as a teenager, Ben dreamed of “making a difference in the world”, not knowing exactly how he might go about achieving such lofty aspirations. He half-heartedly set his sight on a journalistic career. A kind teacher rescued him, suggesting he pursue acting instead.

“The idea of the storyteller who could change people’s lives appealed to me. My father was in the Royal Navy and acting was not something that was taken seriously. But I had this teacher who made me believe that acting was an option for me.”

At 19 he moved to London to join the National Youth Theatre before enrolling at the Drama Centre. Despite an OBE and CV pointing to the contrary, he insists his successes on all fronts are simply down to “being in the right place at the right time.”

“I had the opportunity and I took it,” he says modestly.

And Then There Were None is at the Wyvern Theatre from September 28 to October 3. Visit swindontheatres.co.uk or call 01793 524481 to book.

— MARION SAUVEBOIS