On the No.1 cycle (rinse/pre-wash, whatever you want to call it):
Start the cycle and the machine pumps out any residual water (normal). It then starts to fill with fresh water (normal). As soon as the motor kicks in to circulate the water then the mains RCD trips.
However… start the machine on No.2 or No.3 programme (heavy or medium wash) and it runs through prefectly OK.