At first I thought the problem was just that this thing (whatever it might be) had come off the soil pipe (as I suppose it is) at the back of the house.
But it turned out to be split. In fact, the whole point of the iron thing in the middle was to rust and make it split.
Since neither Wickes nor B&Q nor two hardware stores in Lewisham had anything at all similar, it was a case of bodging up a job with plumbers’ putty:
Further investigation shows no sign of such a thing on the Internet. Maybe it doesn’t exist. Certainly the self-splitting pipe fitting doresn’t sound like such a good idea…