Andrew Gabriel wrote:

Anticipating that advice, I've dismantled it now and found a few rock-hard
black fragments between the sliding chrome-plated surfaces where the
moveable spout rests down against the body of the tap. Presumably there was
once an O-ring there which as you say should be replaced

...lubricate with silicone grease...

Ah, I'll have to get some (and hope that it's available in small quantities)
unless there's an acceptable alternative which I already have.

Many thanks.