thanks for answering. Yes, it always blows hot when in use. I thought it might be connected to the cabin heater, so I've driven with no heating on and it still happens! Ditto other in-cabin electricals. How would you go about getting rid of an airlock?

Thanks for the idea - will check it out tomorrow.

But once you get past about 20mph there's plenty of air blowing through the radiator anyway.

You get rid of airlocks by following the specified bleeding procedure which can vary slightly for different engines.

How quickly does the temperature gauge move into the red? The actual temperature can't shoot up in a couple of seconds, so it might be an electrical fault. The humidity is much higher in winter so there's more chance of moisture working its way into any breach in the wiring loom. Might be worth following the CTS wire back to the ECU and using a liberal dose of WD40 to try to dislodge any moisture.

Hello - I am having the same problem and was wondering whether you found out what the solution was? / what you ended up doing? Thank you!

Who knows? - no response from anyone. Cause looks electrical to me, and I guess as all the posters have found no actual fault, perhaps grin and bear it. It's a feature - they all do that, sir (or at least some do ... ).