Well, we let the 2000 XK8 sit for two weeks during christmas and you all know how they hate to sit in the cold for a few days without getting operated. We fired the car up yesterday, went for a drive and got a charging fault when passenger seat heater was switched on. Turned around and drove straight home, let it sit overnight.... Today the fault is gone (and I'm scared to try the seat heater). Ever seen this before?

First thought is a failing battery with a low charge. How old is the battery? Charge it with a low amp (1-2 amp) charger overnight and see what happens. Or take it to one of several auto parts stores that will do a complete charging system test hoping to sell you a new battery or alternator.

The Battery is a new top quality battery.