Right, JB has a knackered non-return valve, and I assume he is on "snap-on" piping (the blue plastic stuff, 11mm O/D I think) and I have a non-return valve that despite replacement does not work - same piping.

But I have an old-fashioned 10mm non-return valve that works just fine.

Does anyone know where I can get a joint to enable me (and john likewise if he can re-establish piping connections) to put such a non-return valve into modern piping, or get an inline non-return valve to fit the modern piping?