@Alan Stirrup Danfoss heads and valves are very much still available. Does yours have 2 or 3 pipes (the 3 port version sits on a T )
While I have a strong suspicion that the zone valve is part of the problem (based on experience of what is most likely to break) we haven't done enough to confirm this, or prove that it's the only problem. (Also, if the valve itself is stiff, a new head will burn out very quickly, so it might need an entirely new valve fitting)
So before going any further you do need to do some electrical tests. Did you get anywhere regarding some photographs? (e.g. the wiring centre). You need to be sure that the controller is calling for heat, the wall stat is receiving power, and in turn energising the supply to the zone valve.
It wouldn't be expensive to get a sparks in to diagnose this by the way - I'd expect to have an understanding of what is going on in under an hour.