John wrote:

I've unscrewed everything above worktop level and cannot see anything to
fix, although inside the collar with the spout removed there's a gritty
ring about an inch down which is missing a segment to the right. No idea
whether that's design or breakage.

Will the gritty ring be the ceramic disc? The missing segment will be the
variable opening?

I don't know, but on removing the sink trap I've found the rest of it
there, so it looks like that's the culprit.

Replacing the tap is the easy bit. It's removing the massive belfast
sink in front of it that's going to be the challenge, but I'm *really*
glad I used non-setting mastic to bed it rather than adhesive or silicon.