Hi Claire. We had exactly the same problem, twice. Ours blew the trip when the heater switched on, just after wash cycle starts. Best way to test if this is your fault, unplug machine, whip the back off, pull off the two wires going to the heater at the bottom of the drum, (It's under a black plastic cover, and connected by a black and a pink spade connector with a small nut between them) Having pulled the spade connectors off, make sure they hang safely, not touching anything else, keep the dog, kids and granny out of the way, plug in and start the machine as usual. If it's the heater that has failed, your machine should now stand there without blowing the trip switch. Set machine to pump out, and drain water. Swith off and unplug, get replacement heater element, eBay or whatever, Remove earth connector, and little white square connector (held in with a small plastic latch) slacken nut on heater, which slackens rubber bung. Now heater element will pull out, but it will be awkward because if the build up of limescale, but it just pulls out. I think the element is bent either up or down, so make a note of this when yours comes out. Clean up all the gunge from inside bottom of drum, and around where heater seals, push in new element with nut slackened, right way up, and under strap inside drum. When in place, tighten nut to expand the bung, reconnect wires. Test again. If everything is OK Unplug, refit back, and away you go.
Good luck!
Jon W, October 2007