Hmm, not the best scenario to be in up there in Scotland.
Anyway it could be a number of sensors. The first thing to try is to reset the ACC unit. Your handbook should tell you which buttons to press but I think it was the "AUTO" and the vents (bottom right) together. It may be different on your car. If not it is top and bottom buttons on the right hand side.
The system will self check and display any faults as numbers 0,1,2 etc on the display. When this is happening you should hear the air mix flap moving. If it doesn't the mix flap motor could be at fault. This only gives you the number of faults rather than the exact faults. On my system this reset the computer for that trip to normal but my ACC unit was shot so reset itself each trip to freezing! My mix flap didn't move when the box was shot.
You need to then try www.townsendimports.com to get the faults checked. Townsend has some good test procedures for the components. I would think it is probably either the mixed air sensor or the cabin temp sensor.
You can whip the cover off the cabin sensor (under the heated seat switches) and blow out any dust and fluff too.