ALLEN Leech feels he has a wrong to right regarding his crossword ability and that of Keira Knightley, his co-star in the new British drama The Imitation Game.

Telling the story of Alan Turing, Bletchley Park and the breaking of the Nazi's Enigma code, director Morten Tyldum thought it would be a good idea to get his cast in the mood by giving them cryptic crossword puzzles. During the Second World War, this was one way Whitehall had recruited its code breakers.

Leech, like Knightley, Benedict Cumberbatch (playing Turing) and the rest of the cast, was put to work. Knightley said recently that bad as she was, it was some consolation that Leech was worse. Unfair, laughs Leech. "Keira was by far the worst. I'm actually not that bad at crosswords. I just wasn't that great on those ones."

On further probing, the actor -best known to date as the chauffeur turned gentleman Tom Branson in Downton Abbey - cracks. Does he, I ask, prefer quick crosswords or cryptic?

"There's a great one in the Irish Times I love playing called the Simplex. I still don't get it all the way every day but I get most of it."

There's a perfectly-timed comedy pause.

"Then I normally ring my dad for the answers."

With that established, on to The Imitation Game. Leech plays John Cairncross, one of a band of intelligence officers whose work at Bletchley was a matter of life and death. Without the key to crack the code, the Allied forces and ships supplying civilians at home were at the mercy of the enemy. Every day that passed with the code intact was another in which countless lives were put at risk.

A deadly serious business, then, so one would think that the man who led efforts to crack the code (the Poles played a crucial role too) would have immediately earned the enduring gratitude of his country. As the film shows, however, it was not to be for Turing, at least not in his lifetime. After the war, Turing, who was gay, was arrested for indecency and convicted. It was only in December last year that he received a posthumous royal pardon.

Leech's condemnation of the mathematician's treatment is clear. "When you think about what this man achieved and what he did for the British war effort, and how he was so cruelly treated by the authorities and the Establishment, it is absolutely shameful. I think that's why he isn't celebrated because then people will question exactly why this happened to this brilliant man."

When it came to depicting the group around Turing, it was important for Tyldum that they should come together as a real team would.

"He gave us two weeks in rehearsals," says Leech, "which is unheard of nowadays in any kind of production really, except for theatre. Through rehearsals you ended up bonding anyway because you were helping each other create this world and these characters together, which made a huge difference."

As with Cumberbatch and Turing, Leech was playing a real person, something which brings with it added responsibility, he says. Leech did a lot of research, including reading the Lesmahagow-born Cairncross's autobiography. In the end, however, he had to make the call as to how this complex character would be portrayed.

"You always try to find that delicate balance between telling a story in the space of two hours and being true to this man's life."

Once filming began, Leech found the attention to detail flawless. "When we walked onto those sets, it was very easy to immerse yourself in that time. Everything was absolutely perfect, even down to the pencils we used." It helped immensely that filming took place at Bletchley, and in particular the bar where staff went for breaks.

"Walking in there, you got real chills, shivers up your spine. This was exactly where these people were, where all of that frustration, all of that tension, was released." He and Cumberbatch were of the opinion that if the place was dusted for prints today, it might be possible to find those of Turing and his team.

One would imagine Leech to have a good eye for period detail, given his part in one of the most popular television period dramas of recent times. When he joined the cast of Downton Abbey he had a three-week contract. Five years on, he is still there, the driver who went all the way from downstairs to upstairs by marrying one of the ladies of the house.

Leech believes the character has lasted so long because the series creator, Julian Fellowes, sees Tom as a "tour guide" for the show. He is the link between the rich folk and the servants, and with his political background - don't tell the horses, but he is a socialist - he highlights how the times are a changin', even at Downton.

Like the rest of the Downton cast, Leech is sworn to silence over what happens as the current series comes to an end on STV this Sunday. He will only say that Fellowes, as usual, has a few surprises in store. "I'm looking forward to seeing what the reaction is to them," he says.

After five years, the cast have become a family of sorts to rival the fictional Crawleys. Leech is particular pals with Rob James-Collier, who plays the constantly plotting under-butler Thomas. The pair have become known as the "terrible twosome" on set for their practical jokes, one of which was sewing together the trousers of a fellow cast member, Thomas Howes, who played William until 2011.

"We sat outside his trailer and listened as he tried to get into his trousers for four minutes, including four times falling over."

Born in Killiney and a graduate of Trinity College, Dublin, London has been Leech's home for 10 years and it was to the UK capital that his family travelled for the London Film Festival premiere of The Imitation Game in October. A great time was had by all, though his mother was alarmed to be told by her son that she would have to give a speech. Just before they left for the premiere, Leigh confessed it was a joke. Fortunately for him his mother is the forgiving kind.

While Leech clearly likes a laugh, he is approaching a serious crossroads in his career. Having been in a successful stage run of Mike Leigh's Ecstasy in London, plus Downton, and had recent film success in the low budget chiller In Fear, The Imitation Game could see him going upstairs in film, too.

Does he feel the need to choose soon between television, theatre and film? The question is relevant because Downton shoots from February to August, taking a hefty chunk out of the year.

"I will at some point but I'm delighted I'm still able to balance all three. I've always had the motto that I'll do good work anywhere, doesn't matter what medium. That's something I hope I can always keep going. I'm always trying to juggle everything but that's half the fun. It's nice to be busy."

The Imitation Game opens on November 14