The board's sensor seems to be matching up to the internal 4 core temps pretty well (a lot better than the extreme and ud3p anyway). Would the board read the CPU's internal sensors incorrectly?

Anyway left it idling for a few hours whilst ambient's cooled, but still at 42,38,42,42°C and prime instantly goes into 60's / 70's. Starting to wonder if lapping really did me any favours - maybe the block has kind of 'moulded' around the CPU and isn't matching up the the CPU surface any more?!