I think I know this one Duncan. There's a regulator for the fan which has a big alloy heat-sink. It usually fails by intermittently rather than total catastrophic failure. If there isn't a Rover part available I understand Range / Land Rover have identical parts. IIRC, you can repair it though - power transistor replacement.
I can't remember much more about it. Keith Alexander showed me one many years ago.EDIT
- just read the link Mick added - makes this post redundant