Overheating at motorway speeds/high revs for long periods of time

£2000 was specifically because it was a main dealer who had the car for investigation.  It would possibly have been a lot more than that - we all know what their prices are like!

That's a great kit Mike and at £18 plus you get several uses out of it I would say it's a no-brainer, just get the kit and test, it will be quicker than removing the rad or even the plugs.

Good spot

Robin

As ALFS explained on Reddit those testers detect CO2 from cylinder pressure and work by bubbling gas from the radiator through the fluid, as it dissolves it becomes acidic.

(I actually thought you tested the coolant for hydrocarbons, as testing the coolant itself for acidity is never going to work)

I'd save the money and just look for residual pressure in the radiator when the engine is cold or the level in the expansion tank going up as I suggested over there.

I also suggested looking for signs the crank pulley is new, as when the pulley de-laminates and the belt comes off, some owners drive it until it boils up, which can warp the head and cause a weeping head gasket..