Hiya,
I found the hot water cut off valve on mine had disintigrated on the inside and wasn't, well, cutting off the hot water. The thing looked OK from the outside but did nothing on the inside. The valve is in the engine bay, to the right of the battery and under the wiper motor. I couldn't get a jag part so went scrounging around the breakers and found one on a Renault or something. My Granada has the same part on it as well.
When the heat is switched off you should see the valve operate, just to see whether it may be this or the vaccuum that operates it is missing.
pottsy