I borrowed an infra red thermometer, took the car for a good run, the auto meter temp gauge read just over 240 degrees and the fan did not come on. Took the dipstick sender out and touched it, very warm, but not excessive. I then measured the following temps with the thermometer:
Heads: 240 degrees
Oil filter: 170 degrees
Block: 175 degrees
Kept the car running (ticking over) to get warmer and after a few minutes the fan kicked in!! All the readings had then gone up about 10 degrees. Turned the car off and after a couple of minutes the fan switched off - working exactly as intended!!
So looks like after a certain temp, the sender/gauge goes way out of calibration - so the bin it is!!
that's good news... at least an easy fix.