So I called the boiler people, spent thirty seconds confirming who I was, they confirmed the boiler was still under warranty, and set up an appointment for tomorrow morning.
Julie can cope with the electric fire for one more day, and hopefully it _will_ be the valve, in which case they won't have to take up half the flooring, and it will all be over soon.
(You have no idea how much I dread the flooring being taken up. Two lots of living in a completely disrupted flat was quite enough.)
Original post on Dreamwidth - there are comments there.