As per title...
I stripped down the front end of the car this morning, then put things back together (in what was the right order for the job I was doing) to find that afterwards the outside temp wasn't showing on the dash, just a blank space and "deg C".
I assumed I hadn't plugged it back in, so was puzzled to find I had.... scratched my head a bit and did some googling. Turns out you have to plug it back in before reconnecting the battery or it has a wobble. So I popped out, unplugged the battery for 10 mins, plugged it back in and hey-presto works again!