Don't ask me how, but the Bomber's mate Jonnie managed to mess up the flushing mechanism on the upstairs toilet last night.
I stripped it down and shut the water off, and I've been doing a little online research this morning; it looks to me as if I will have to replace a Gerberit Impuls260 dual flush valve.
It seems that Facebook's heuristics have been misled by my intense interest in faulty plumbing however; they're promoting a "discreet male catheter" to me in one of the adverts on the left of my stream. Stream! Geddit? Geddit? Funny!