I have exactly the same problem. To check your heater measure the resistance across the terminals with the connector removed, should be approx 26 ohms.
Having spoken to Zanussi service they advised to check the temp sensor which screws into the bottom of the dishwasher in front right corner (2 orange wires to it), they couldn't advise me of the resistance but it should have a reading of some sort other than open/closed circuit. I found open circuit on mine - hopefully this is the cause.
Mark, December 2005