Thank you for taking the time to give me some advice.
My gas fitter is really nice and helpful, but he said himself, as the fault is intermittent, he could not be sure it was the fan playing up but as the wiring seemed OK when he checked it, the error code given meant it was probably the fan. I will mention the pressure switch to him. It seems strange that it works fine just for the central heating, but the problem just seems to happen when running the hot water.
Yvonne
January 2013