This is one of those things that has been bounced around in Doctor Who circles for a while, and I am going to try to get it all straight.
Take a look at this picture:
The guy in the back with the handlebar moustache is an actor named David Baron.
In the sixties, when he still worked as an actor, the playwright Harold Pinter was registered with Equity under the stage name "David Baron."
Is that really him? As far as I can tell, the jury is still out.