Astonishing news - my computer still works
Took a bit more force to unclip the existing heatsink than I thought it would - good job there's nothing delicate on the mobo next to the securing clip that might not welcome the attention of a screwdriver
And I have to unscrew the power supply and move it out of the way to get the leverage.
But against all expectation, when the new one was in, I turned on the PC and .... it worked

Temps seem to be about 10 -15 degrees lower now - I'll see if it keeps that way.
Thanks for all your help with this