.... But alas, not a nice simple one. It's a type I've never had to fix
before. Hot water side developed a fairly sudden leak.

The tap assembly is less than a year old - and a "Tapmate Narran" (
Details here http://tinyurl.com/6kwovn )

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.

My biggest problem is that the belfast sink has to come out to get to
the underside of the tap, and I'd rather not do that until I've got a
replacement tap or a clear way of fixing it.

So - how do I fix it please?

(The tap is actually less than a year old so a replacement should be
gratis (B&Q are usually ok about that sort of thing))