Eventually (today) we managed to get hold of a plumber – Handsome had to stay home from work this morning so that he could let said plumber in and show him the relevant leaky bit. The plumber (wonderful people, plumbers) pulled at the leaky bit, and the whole pipe disintegrated. So, we now have a completely new pipe, a hole in the floor (currently covered up to prevent the mad cat from going exploring under the floorboards) and a worrying nagging feeling that perhaps we should check out the rest of the pipes too. Maybe after Christmas!
Now we just have to wait for the builders.