I think I'm on verge of solving this through work myself and WIKI. The story so far......
As I arrived in Devon for Easter coolant light starts flashing. On return journey after top weekend overheats in traffic and get towed home

Get back and have a look at the thermostat - it fails to operate in boiling water. That explains fan not kicking in...I think. New thermostat fitted - I fill up resevoir to brim, run the engine and repeat process. No air is coming back into resevoir. Happy days I think so head off to Gower. 14 miles later light is on again and the needle is 3/4 towards overheating. Again the resevoir is low, no leaks and filler tank is full and venting through top.
The heater throughout the 14 mile is working then not etc.
Having read tthrough WIKI I reckon that I just haven't filled coolant back up properly and I haven't bled it at all properly!
Now have a half full reservoir, a 3/4 full filler tank - where should I start the bleed / filling process (not sure if I have over or under filled it).
Any advice greatly appreciated
Jack