Mousetrap celebrates 25,000th show with celebrity cast Downton Abbey star Bonneville will play guesthouse owner Giles Ralston, with Greig playing his wife Mollie. Iain Glen will play policeman Sergeant Trotter, with Hart appearing as guest Miss Casewell. The sell-out performance will see all proceeds going towards Mousetrap Theatre Projects, a theatre education charity which receives funding from the play. The writer's grandson, Mathew Prichard, who was given the royalties for his ninth birthday, but has gone on to donate them to good causes, will welcome the audience. Prior to Sunday's version of the show - directed by Phyllida Lloyd, whose films include Mamma Mia! and The Iron Lady - a permanent memorial to Christie will be unveiled at nearby St Martin's Cross. In September, the first ever UK tour of The Mousetrap began, which is due to continue for 60 weeks. When the production opened at the Ambassadors Theatre in 1952, Lord Attenborough and his wife Sheila Sim took the lead roles. It played there for 21 years, moving to St Martin's Theatre next door in 1973 without missing a performance. Over the years, more than 400 actors and in excess of 200 understudies have worked on the play.