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 *******s with no standardisation,
but this one has 32 splines - which turns out to be a very find pitch.

Even https://www.tapmagician.co.uk/TMdefault.aspx who sell everything,
don't have these...

Very depressing - how many variants does the world need...