I have a 20 year old Potterton Netaheat Profile 80e.
Boiler heating water OK all summer.
Wanted CH on today but turned control thermostat setting down first (from MAX) as a length of pipe is buried below some screed that was only laid last week.
Having turned the control thermostat down the boiler no longer lights even when there is a call for heat (confirmed by the fan running).
Boiler no longer lights under any circumstances - whether call for heat from hot water or from CH and whatever the control thermostat is set to.
I have pressed the reset button and also removed all power from the boiler in an attempt to 'reset' it.
When turning up the control thermostat there is a single click from something in the boiler - I think it used to do this before but mention it in case it is important.
Many thanks for any input.

John Preston, September 2010

Update for anyone interested.
The two pressure sensor tubes in the flue area of the boiler behind the fan had both disintegrated - conincidence that they fell apart when I changed the control thermostat setting ? - who knows.
Anyway have replaced these with temporary silicone tubes while waiting on the proper spares from Potterton and everything working OK.
FYI the required spares are 212540 red tube 350mm and 641218 vacuum tube 350mm (and NOT the following which are what most part suppliers say you need - 641270 red tube 140mm and 225536 (was 212496) vacuum tube 170mm).

John Preston, October 2010