Well, fixed the climate control this morning. Changed the External Temp Sensor, mounted on the NS fog light bracket.
The problem:
Car jacked up, wheels left ON, under-tray taken off... Disconnected the plug.
You see that patch of water on the ground? This is the water from the plug/sleeve that didn't land on my head. Serious water ingress here.
Plug cleaned & WD-40'ed, problem still the same.
Unhooked the NS of the bumper and got my hand in to unhook the sensor. It isn't bolted on so you could probably do this job without releasing the bumper, it just slips between some grooves.
Attach the new sensor and test with it just dangling,
Job is a good'un.