Worcester 350 - Fuse problem?

I currently have the worcester 350 combi boiler and for last 2 weeks it keeps blowing a fuse. we've had 8 call outs from scottish gas who have replaced numerous parts but this doesnt seem to make a difference. It is blowing the fuse inside the boiler but has also blown the main switch as well. It seems to go mostly during the night and has never lasted longer than 23hrs.


The parts changed so far are:
Fan, Fan Resister, Pump, Circuit board, driver board, PCB, heat switch.

They are now going to replace the pressure switch and timer clock but I doubt it will make a difference.

Does anyone have any experience with this that can help?