Do you have a motorised valve in your airing cupboard?
Ok, silver or white - 2 or 3 pipes?
It should be a square box sat on the pipework with a wire going to it?
is there a lever on the end of it, sticking out?
Can you turn the heating on then can you give the lever a slight push and tell me if it is stiff or slack?
Ok, when the heating is on it should be slack showing that the valve is open.
If you push it open and latch it, you will get heating while the hot water is on
You will need to get a new motor fitted or replace the head to cure this
Yes, but you will be able to get your heating on until they come now