Pic of a Spit used in high speed dive trials 27th April 1944.
Dive resulted in loss of reduction gear and prop.
A previous succesful dive resulted in a speed of 606mph, mach 0.891.

Another claim, generally thought to be inaccurate due to the measuring equipment was a Spit F.19 in 1952.
Climbed to 51,550ft (highest ever for a Spit), but the cabin pressure fell below safe levels.
In his haste to reduce alt the pilot lost control, finally regainig control around 3000ft.
Evaluation of the equipment onboard suggested he reached 690mph or mach 0.94.
But as I said at the start, generally thought to be inacurrate.