I've been lurking for a while without registering and want to first say thanks to all those who have posted about the two problems I've had with my '07 A3, and also to pass on some info regarding one of them. I've had the coolant level sensor warning light (but the level is OK) for a few months now, and also have had intermittent problems with my reverse light.
Onto the coolant header tank; basically what I've done is firstly short the connector out with a paper clip to see if it solves the warning light, which it did. I then took the tank off (two screws and a couple of pipe clips) and blocked off the lower hole. I then poured in half a bottle of white vinegar and angled the tank (using a mug as a kind of pedestal!) so that both probes were totally immersed. I left this for a few hours, then gave the probes a rub with one of them wooden stirrers you get in coffee shops and rinsed the whole thing out. This appears to have solved the problem and has cost me about 50p and half an hour of my time.
Thanks again for whoever posted about this problem, it's saved me a lot of cash.