I went and prodded the tank, which told me it was 15% full. I thought this dubious given that it was last filled in October, so I rang Calor and asked them if the gauge might be off. They said that Telemetrics had told them the tank was 14% full, and passed me to Emergency.
I was very impressed. Emergency told me to find the Regulator Spindle and twiddle it. Unfortunately our tank is rather venerable, so their description of where the Spindle might be hiding did not match our equipment. They sent an engineer instead, who turned up, to my amazement, within three hours, and twiddled our spindle for us. For free! I was even more impressed when he got the gas flowing again. The fire and boiler both had hiccups to start with, but seem to have resumed operations now we have turned them on and off again a bit.
He has promised to send a new Regulator, as ours is apparently over 20 years old (I would have guessed at least 30, myself...). Calor may not be a cheap way to heat a house (we've just gone up to payments of £125 a month! Woe!), but they certainly seem to do a good job for the money. I can definitely see why people are putting wood fires back into their houses though.