Anyway, there's been a series of problems ever since. The boiler is understood to be 100% OK - as confirmed by the Vaillant engineeer who has visied twice.
The symptoms are:-
1. Recently, the circulating pump (adjacent to the HW cylinder) appears to be running continuously - even when the programmmer is not calling for heat. It is set to operate to provide central heating and hot water during two time periods per day. The radiators warm up and cool down exactly in accordance with the programmer schedule. The boiler fires up every 3 or 4 minutes and when the boier display reaches 68 degrees it stops heating. The temperature in the display drops slowly to around 45 and then the burner starts up again. This cycle carries on 24/7 - even when the programmer is not in an active period. Presumably it is trying to heat the HW cylinder as the radiators stay cold when the programmmer is not in an active period. This problem has emerged only recently.
2.If I over-ride the programmer during an active period ie to shut off the heat, then the boiler stops OK but will not restart later on theschedule. It registers an "overheat" situation and has to be reset by taking off two cover plates and pressing the reset button. I believe this is because the pump stops and hence the circulations stops - causing the combustion chamber to trigger an over-heat situation.
Factoids:- After the boiler was first installed, there was no display on the boiler when the programmer was inactive. I complained about this and the builder brought in an electrician who connected a permanent mains supply to give a fulltime display. We thought we were over all the problems the problems then but a few weeks later problem 1. above emerged.
The questions that spring to mind are:-
1. Have I recently developed a a stuck motorised valve? 2. Did the plumber wire up the programmer correctly to the boiler? 3. What can I do to isolate/identify the problem further? 4. Where can I get a competent boiler installer cum plumber in South London that can check this out.