"I am prepared to be told it can't have happened, but I trust my memory."
I'm guessing that you're right and that the official versions--"first public demonstration," "first official sonic boom," etc.--are wrong. Been known to happen. Some guy put the nose down a bit and did the ton, just as he had the week before, for the fun of it, out over the Channel or whatever. Nobody will ever -prove- it, but you were there.