OK, I cant for the life of me remember where I read/heard about spitfires going supersonic during dog fight.
I would say the confusion stems from the fact that towards the end of the war several fighters (not just the Spitfire) had enough power to run into
Mach effects in dogfights, such as trim changes, buffeting, or even loss of control, caused by the airflow going supersonic
locally, without the aircraft itself going supersonic.
I'll have to find a source for the max. Mach reached by a Spit in a dive. This was after the war, as part of high speed research.