A toilet.
The syphon diaphragm had split, leading to increasingly intermittent flushing.
However, because it's a Close-Coupled bog, to gain access to the bottom of the cistern ( where the locknut wot holds the syphon in place is ) requires removing the entire cistern from the bog.
This in turn requires removing the clamping screws wot do hold the cistern and pan together, which are of course rusted to buggery.
So it's dremmel time to cut the buggers off.
Then cleaning up accumulated years of disgusting eek and a great excess of Plumbers Mait putty.
Replacing the syphon unit itself is trivial.
Then an emergency visit to B+Q on account of the fact that the Close Coupled Kit ( Replacement big sealing rubber washer, and some new bolts ) I'd previously bought didn't fit.
All back together and seems to be water-tight so far!