I am answering my own question so it may help others.
I tested the resistance of the heater. 26 ohms. I know that is ok from information received. I had a spare logic card so rather than attempt to meter the heater circuit with the 230v on, I swopped the logic card. Fixed it. So, a defective logic card was the cause.
I expect the error light only indicated 'heating failure' and not the specific faulty item.
Nigel, May 2012