Here is exactly what I'm experienceing with my Britony 80 combi boiler.
CH problems: Boiler is not firing up very often, and when it does, it switches itself off after a few seconds - or after 40 about seconds if I'm lucky. I have disconnected the programmable thermostat/timer to eliminate that from the equasion. I bridged the contacts where its 2 terminals were connected to the boiler's cirquit board, so that the boiler is now conrolled only by the TRVs on the rads, a couple of which are fully open.
DHW problems: Turning on a hot tap produces plenty flow, but the water is cold. The boiler fires up for as long as the tap is left on, but the secondary heat exchanger stays stone cold. The first sign of trouble was several hours ago when the tapwater only got luke warm. Now it is stone cold.
The system was working satisfactorily a few days ago. The first sign of trouble was when the CH system burner didn't fire up as often as I think it should have.
Thanks for any suggestions,
Jim D