Thanks for that, I'll spend some time on it this weekend. At least I know where to start looking.
If I understand you correctly, both fans are supposed to run slowly, all the time with the A/C's operating, running at full speed only when the engine reaches this high temp.
I'll try to find the relay, but A/C over here is a relatively rare option and the manual I have doesn't show any of the A/C electrics.
Englands weather being predictably unpredictable, it's turned cooler now, and the problem gone away - for now!!!!