I think it's William Mervyn
I don't know why, but he appeared in Carve Her Name With Pride as an M like character, but he appeared to be dubbed by Geoffrey Keen. Maybe someone else can confirm it. He did use his own voice in a small role in Dunkirk
He was the tv show All Gas And Gaitors and played the old gentleman in the film version of The Railway Children
He also regulary played a Judge in Crown Court