Hi Andy looking at 12 x 10 min Darks taken over two hours the mean value was 2664 +/- 1 ADU after hot pixel removal. If there were any drift in the Offset/Bias I would have expected a bigger variation and or trend.
I have observed that if you take a sequence of bias frames the temperature of the camera shifts slightly. In order to counter this I add a delay between taking Bias frames of 10s to let the camera re-stabilise. It may have no impact on the but I did not want to take any chances.