I believe I have a coolant leak. I've gone through about 3 litres (measured when refilling the system) since November. Last month the car had a service and the guys at the garage spent a long time trying to find the source of the leak, or indeed any evidence thereof. They found none (high pressure testing carried out, water pump checked, ...). In the last 6 weeks I've not added any coolant at all. Is it possible for a leak to seal itself? Am I looking at a the joy of a catastrophic failure in the near future? Has anyone else experience anything similar?

It's not unheard-of for the filler cap itself to be a bit leaky - especially if there's crud on the O-ring. Have a look for any pinkish-white crystals around the top of the expansion tank... If a clean of the mating surfaces doesn't sort it (may have been what the garage did for you!), a new expansion tannk costs about £12 (you can't get the cap on its own AFAIK, but it's not that expensive for the whole lot, I guess.)