To bring this threat to a conclusion, it was the correct compressor. Identical to the OE one - or at least the clutch was.
It was a pleasant surprise that the clutch was relatively easy to swap out with the compressor in situ. The only tricky bit was stopping the pulley rotating while unscrewing the retaining bolt. I had to use vice grips. Once the old clutch was out, it was quite clear what the problem was:
The rubber "spring plate" has melted and detached from the centre of the pulley.
I used the clutch and the shims from the new compressor which gave me the same 0.5*mm air gap. I suppose I've got a spare compressor body for future use if necessary.
Anyway, the AC is blowing ice cold again and the car seems all good. Happy days.
Thanks for all your input.