Netaheat Profile 80e not lighting when call for heat?

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