Hugh Bonneville and Julie Walters were also among the cast for the special show at St. Martin's Theatre in London in honour of the 25,000th performance of the play, which premiered on 6 October 1952.
However, Sir Patrick admits he had never seen the world's longest-running stage production before agreeing to take part in the celebration, which benefited the Mousetrap Theatre Projects, a charity dedicated to bringing the magic of theatre into the lives of young people.
He revealed, ""I came to see it last night for the first time, and I thought 'How has this passed me by?'""
Downton Abbey star Bonneville added, ""Tonight was the 25,000th performance and tomorrow will be the 25,000th professional performance, so that is pretty special. I'm very proud to be part of such a great legacy. I think she (Christie) has given work to all of us over the years.""