A couple of minutes later and it went back off I have left it running with a digital thermometer in the van. It was reading 14.5 degrees when I left it and I will nip out occasionally to note the variations in temp (plus the thermometer has a "min and max" recorder).
One thing I did note that might have had summat to do with it was that the thermistor (small grey thing just to the left of the main stat adjusting shaft) was bent over, so was lying very close to the pcb instead of sitting upright and therefore in full "view" of the vents under the cover. I bet that was half the problem.
So far I think it's a "result".
By the way, what with the new "SAFEFILL bottle I don't have to even think about the cost of gas any more