Surprisingly difficult to discover, though as he has a son Joe who is also an actor it's reasonable to assume there is or has been a Mrs Armstrong. He seems to be a very private man and I can't discover any biographical details apart from his date and place of birth.
As for "Who is he?" - only one of our leading stage, film & TV actors, one-time member of the Royal Shakespeare Company and Laurence Olivier Award winner (don't tell me you've never heard of Laurence Olivier either...??)