Got a problematic Potterton Netaheat 10-16 boiler.
It will only light when it's been off for half an hour
or so. When I start it from cold, the pilot lights
after about ten seconds and then the main burners come
on about 40 seconds later. This is all as per the manual

However, when the thermostat cuts in (i.e. turns off the
gas flow), the boiler will not relight [1] until it's been
turned off (at the wall switch) for a while. Even if
it's cold (i.e. hours have passed since the main burners
were running) it still seems to need turning off at the
wall switch for at least five minutes.

Any ideas here? The service bloke seems to think the
thermostat is at fault. The neighbour's suggested the
mercury vapour switch. I think she might have more Clue
than the Corgi-reg'd service engineer...



[1] at this stage the pilot light is off.