After a bit more investigation it's clear that the thermostat is all over the place. Also I think the heater matrix isn't getting enough flow when the thermostat is open? As it if left on hot, it blows nice hot air for a bit and then goes cold. Then it will come back again.
This all seems centred around the thermostat. But I don't understand why they work for a bit and then break?!
The thermostat housing isn't a focus rs one, I think I took it off a 2.0 mondeo or something. But I can't see how this would make a difference as the thermostats are all the same?