Phillip Brown wrote:
Just to update the thread, I tested the fan on separate mains and it ran
fine. Turned out the air pressure switch was stuck, which the Potterton
troubleshooting flow chart pointed me to. Cleaning it out resulted in a
working heater.

Are you saying that because the air pressure switch wasn't working the
fan wasn't coming on? That's puzzling me because AIUI on these boilers
the fan runs whenever mains is applied regardless of the air pressure
switch, but the latter has to operate to make the boiler light.