Graham. wrote:

The shower on the first floor is leaking slightly,
causing water to drip into the kitchen below,
through a crack in the plaster.

I'm wondering what the likely cause of the leak is;
I assume it is somewhere in the drainage from the shower,
since the shower itself has a single base (plastic or ceramic)
which does not seem to be damaged in any way.

Run the shower directly into the drain for a few minuets to prove
that. With mine it is hairline cracks in the wall tile grouting.

Thanks for the suggestion - I should have tried that.
Hopefully it is hairline cracks in the grouting, as you suggest.

Luca the plumber promises to come tomorrow.
He installed the shower, so maybe will feel he has to mend it for nothing ...