Am trying to sort out a shower valve which now only gives out cold water.

This is a Wickes own-brand mixer, not thermostatically controlled, is
fed by a combi boiler, and formerly worked fine. Having removed it from
the wall I've established that HW is definitely reaching it, so the
problem is within the device and not the supply.

It has one knob, which turns on the flow and progressively increases the
temperature as it rotates. It's apparent that the flow from the shower
head actually falls off at the point when the water should be getting
warmer presumably due to HW not being dialled in when it should be.

Pic of the beast is he
http://img701.mytextgraphics.com/photolava/2008/06/15/img0698small-4auy0zgp9.jpeg

I've already had most of it in bits and reassembled it in the hope that
something that was stuck might have got freed up, to no avail.

Have to admit I don't really understand how it works - there seem to be
several pistonny bits and sliding seals in there. Nothing looks
'broken' as such.

Has anyone come across one like this, and/or know what the problem might be?

Thanks
David