I appreciate it is the bank holiday, but as my house is rather cold, I hope
you can respond before then I recently fitted a reconditioned Potterton
Netaheat Fan Unit after the old one gave up the ghost, and it has be running
well until I turned the thermostat down for one night whilst I was away.

When I returned on Sunday, the thermostat had turned itself on as the house
was below 21-degrees, but the boiler wouldn't turn on. Nothing I could do
would make the boiler start up again. I have tried:

Replacing the thermostat
Testing the manual control of the motorised valve
Turning off/on all the switches on the timer
Checking the fuses to the main system and on the boiler's PCB

When the thermostat turns on, the valve opens and the pump starts, however
the boiler just does not ignite. It also does not make the 'humming noise'
that you get for 10 seconds before ignition, which I assume is the fan?

Is it possible the fan could be faulty, or is there some other possibility
between the timer (which works as it too turns the pump and valve on and
off) and the boiler?

Many thanks in advance of your prompt reply.