Just for information, I fixed this issue and it was the microswitch.
First off the problem got worse, so turning the boiler on, the domestic hot water was switching on right away and constantly (the switch must have permanently stuck). Though there is no danger as flames turn off after a...
I was thinking the same (it's a cheap switch/diaphragm module and can be changed really easily - by myself).
But I just spoke to the guy who I bought the switch from last time and he was thinking it might be the electronic PCB board (I recall 5/6 years ago British Gas engineer replaced that...
This morning I heard clicking and went to the boiler and noticed the domestic water heater switches on and off - without anyone using the hot (or cold) water tap !?
I switched the unit on and off and opened the tap a bit and even turned on the radiator. But now as soon as the boiler is...