Faulty Sycron motor in valve to heating downstairs It could be in the airing cupboard and could be a silver box on the pipework or white please take a picture for
Me to be able to show you which one it is
The programmer you have will need to be completely reworded as that programmer has all the wore centre in it so the starting point would be to remove the old programmer and fit a wire centre. Next thing is options you need a 2 channel programmer ( hot water and heating) and a thermostat you can...
looks like clearances will be wrong on that and if that is inspected it would fail. * dont you have to have a access point on them ? (i dont fit wood burners)
If it's quarter turn with a lever style handle the lever might unscrew. This then will remove the head or show a grub screw under it that will need removing before the head will come off , then follow the instructions everybody else has given you.
Madrab is wrong you can work on gas In Your own house but it isn't advisable as most home insurance becomes void and you have to prove you are competent to work on it if anything goes wrong
Faulty pcb ( chattering relay) get a gas safe registered engineer to check the system over to make sure it's nothing else causing the pcb to buzz. The. He should be able to replace the pcb for you.
Steel is better but doesn't keep the heat in the same as plastic all baths move a little when installed due to the weigh added to it when full I would go steel or castalike (very thick plastic weights the same as cast comes with 25 year warrenty