Thankfully the problem turned out to be simple. Apparently there's a pressure vessel in the boiler which needs to be kept pressurised. If it drops too low then it can't balance out the pressure in the central heating system, which then rises too high, the vent valve tries to remedy this, gets stuck open, and all of the water goes out of a small copper pipe into the back garden. The pressure vessel needs to be pumped up once every couple of years.
Which frankly sounds like a bit of a racket - as you have to get an engineer to do it, so that's a visit just for that every couple of years, for the life of the boiler. I also discovered that they do boiler insurance (which I'd rather have anyway) and that this includes a yearly maintenance checkup, which would sort this. And they're priced competitively with British Gas, who I was using with the old boiler. So that may well be the best option.
In other news, I went to see Hugo last night with tisme and momentsmusicaux, and it was amazing. Fuck James Cameron, Martin Scorcese is the master of 3D movie-making. The film had sense of wonder and magic that I've seen in precious few movies that actually _have_ magic in them. Highly recommended.
Original post on Dreamwidth - there are comments there.