HELP! Hotpoint wm51 fault diagnosis?

Hotpoint First Edition 800, WM51
when power is turned on machine is fine for around 12 seconds then knocks out mains tripswitch. It is hard to tell exactly, but it sounds like the machine trips out a fraction of a second after it opens the water intake. The machine has been acting up slightly (door catch on registering a closed & occasionally going dead halfway through a cycle, but fine when unplugged and left for a few minutes). I have replaced the main power cable as a break in this was suspected at first. Any help welcome - i'm a complete mechanical moron.