The colour changed to green, indicating that gases are leaking into the coolant. The garage claims they can't determine if it happened three days or three weeks ago. Now I have two options: either search for another garage that is willing to fix it, which will cost me over £1,400 (or 70% of the car's original price), or try my luck in court with scant evidence, such as the fact that the mechanic has flushed the radiators and coolant and there's antifreeze on a couple of receipts, and his firm denial of any head gasket problems. My head gasket has definitely blown.