My expansion tank in the loft over flows only when central heating or water heating is switched on(gas central heating)

Is it an old style in the ceiling or is it a light tan/ grey bottle looking thing on the pipes?

If it is a newer style than your bladder inside might be bad. There should be a valve on the end that looks like a tire valve. It might have a cap on it. Try to push on valve with a screwdriver. If water comes out than bladder is bad. Only thing you can really do is replace it.