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No cold water tank, sealed hot water cylinder - unvented system. On
every other outlet both the hot and cold seem to be working at
mains-type pressure. Since there's no pressure difference between
hot/cold anywhere else I guess it could be some debris in the mixer
itself. Will turn the water off again and investigate.

The only reason I'm wary is that the entrance to the inner sanctum of
the mixer unit is via for of those 'star' screws. Which has always said
to me 'you don't want to be doing this unless you know what you're
doing'. Ah well!

I took my shower mixer to bits, it had torx screws, but inside it was
actually very simple, so I wouldn't be too scared. Mine has some mesh
filters actually inside the mixer, so it could be you've got something
similar with some crud in it.

It certainly sounds like a hot/cold pressure imbalance for whatever
reason.
--
Tim Mitchell