Hi. Well here's the current situation. Decided to try and bleed the system today, loads of air in system. So it's all bled out, now the heater is working fine. Just done a 50+ mile journey with heater on full blast to make sure I had circulation, all the time kept an eye on the temp gauge. No problems at all, gauge varied from the first mark to the dead centre, depending on whether I was motoring or stationary.
So, from that I can assume the thermostat, water pump, radiator and heater matrix are working, also the valve must be working as well.
It is possible that I let the coolant level get too low, allowing air into the system. Every time I had checked the level in the past it had never seemed to move, so I kindof didn't keep an eye on it like I should do.
I could have a leak that is too small to detect at present, I will have to keep a close eye on the level and see how it behaves. I suppose there is also a chance that the head gasket has failed and is weeping water into one of the cylinders, though the engine is running perfectly and the cooling system isn't pressurising.
The car has done over 120K miles so I suppose I should expect things to start failing, I just want to know what has failed/is failing. I'm ok with doing any work that is needed, just want to make sure I don't spend on unnecessary parts.