OK - that was a bummer...
4 new 1/4 turn lever taps, less than a year old. One failed (dripping).Circlip holding gubbins together was rusted away.
Other 3 taps all have copper coloured circlips, with no corrosion.
Supplier will replace - but I had already taken it to 3 plumbers merchants plus Tool/Fix and no joy.
Cartirdges are well known for being bastards with no standardisation, but this one has 32 splines - which turns out to be a very find pitch.
Even
Very depressing - how many variants does the world need...