I did some research. Mr Pettigrew was played by Arthur Howard. Howard (born Steiner) was the brother of the film star Leslie Howard and the father of the Shakespearean actor Alan Howard. He was a busy actor, routinely cast in supporting roles as vicars, schoolmasters and civil servants. He was in Passport to Pimlico. He was in Moonraker. In 1961 he spent a week in prison for "soliciting for immoral purposes". He died in 1995.
His performance as Pettigrew survives in the spin-off feature film Bottoms Up- which I haven't seen. When the show was briefly revived in the early 70s (those tapes still exist) Howard had been replaced in the role by Julian Orchard.