thanks heaps guys for very thorough answers - but compressor is still engaged until I switch it off. No Compressor did not die - was just weak when replaced, and this was happening before replacement. Tacho, Serp belt and cruise control are fine.

Rick when you say monovalve do you mean the big one behind the middle of the dash? I can hear something slamming shut reassuringly when I turn the fan off ( cabin fresh air fan) But I HAVE wondered if all those valves back there are working.

Bloody intermittent problems can drag on for years!