Thanks a million for the advice.
The blockage was not in the wc connection but the main house drain.
Luckily your advice alerted me before the situation became too serious.
I owe someone a pint.
Standard S trap rimflush wc pan, mounted on tiled concrete floor.
The level of the (trap) water in the pan fills to normal after flushing but immediately drops significantly. The system has worked normally for years before this problem.
There is no sign of leakage and the outlet fits into...
Worcester Heatslave Highflow.
I have two that need replacing.
Elmwood 2455R
Elmwood 2455RC
Plumbcentre sold me the one supposed to replace the 2455RC, labelled Worcester Bosch Stat Overheat but without any type nos.
They declared the other to be obsolete and no longer available...