Alright, disabled Tasker. The results are not that encouraging. There
shouldn't be anything else starting up when I plug it in to charge, unless my ROM is automatically doing things with e.g. CPU speeds, which it shouldn't be since (IIRC) it's a stock ROM.
Will try it with my old HTC charger and see if that changes anything!
I suppose only other thing is I have Bluetooth on (not connected) but that shouldn't affect it - or it would be affecting the drain times off battery also.