Coolant mystery

[Bowline5]

Six months ago I had a timing belt and water pump done on my 2.5l turbo engine (EJ255). Two days ago on a short trip the engine started to overheat, nearly up to the red, whilst the heater blew cold air. I got home, let it cool down and checked the coolant level, a bit low in the expansion bottle but the radiator was full. Today, halfway through a 10 mile journey, the same thing happened again. If I kept the revs at 1500 or less the temperature gauge stayed below the red line and the heater eventually started to blow warm air. Could it be an airlock, a sticky thermostat, a failed pump, a blockage? Any ideas or suggestions most welcome.