Recently had a central heating problem which required the cold feed to
the hot water cylinder being shut off. Found that the hot water tank
would still magically fill up. Seemed to be coming in through the pipe
at the top of the cylinder which would feed the hot water taps.

It was confirmed when the cylinder was drained and cold feed to it
turned off.
(ie no hot water should flow to any taps.)
Turning on the hot water tap in the bath and water would run out.

Traced to the shower mixer. Which seems to be leaking cold water into
the hot side of the mixer and then fed back into the hot water cylinder
and also the tap as it's connected there too.

The shower mixer is one of those victorian (imitation) mixers built
into the wall.

What I need to know is does it require a completely new mixer (£300
plus the cost of fitting so effectivly £500) Obvioudly dont want to do
that.

Or is there a DIY fix which I (with moderate experience) can try.

It's a problem as clearly cold water is trikling into the hot water
tank of it's own accord 24hrs a day.

A.