Vaillant turbomax VUW/GB282/1E heating fault?

I was called to take a look at this particular model earlier today and whilst waiting for a response from Vaillant thought I may test the local knowledge. We have a situation where the bolier will fire perfectly satisfactorarily for heating and hot water and switch between the two without fault, however once back in heating mode the moment the timeclock is turned off or room stat. or boiler heating temperature thermostat is turned down the boiler fails to relight when there is next a demand for heating. The hot water still operates but then after the boiler returns to demand for heating light on but no further start up sequence. Any ideas other than a suspect P.C.B or is that a cop out attempt at a cure ?