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Interesting read there Clive but I think your assessment of the mirror temperature is erroneous. Anthony Wesley has shown that large mirrors can cool at varying rates on different part of the mirror. His data shows this well. The imager would be picking up surface temperature and not the core temperature of the mirror you imaged. The core temperature will continue to release all night without active cooling. Using a mirror fan will help to some extent, but you cannot get a large mirror to within 0.5 of ambient by simple fan cooling.
My suggestion would be conduct the experiment again and this time use two temperature sensors one on the mirror outside edge and one near the center of the mirror. Each sensor should be isolated by foam with silicon sealant. That way the sensors cannot interact and read the outside temperature. Data log the data from these sensors and compare the information against the IR imager. I think you will get a vastly different result.
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