can you isolate the system and leave just the timer connected, try taking out the cables to the heating and the hot water one at a time and or both at once.
hope this makes sense as I am a electrician not a plumber
Make sure your switch is x-rated
I think it means that it is suitable to use in a commercial environment where you would have lots of fluorescent lights.
I refited my kitchen a few years ago,we have one centre lgt plinth lgts (not yet fitted) and under cupboard lgts.
The 3 sets of lgts are fed from a 3gang switch, where I have the Live and Neutral in and then the Live and neutral out to the 3 sets of lgts