"Matt" wrote in message
om...
Hi, I wondered if anyone had a suggestion to help solve my shower
problem...

My flat is on the top (2nd) floor, with the cold water in the bathroom
fed from a communal tank in the roof. I have a combi boiler that
provides the hot water at a decent pressure.

snip
Whilst I could change the supply to the bathroom so that it's mains
fed, I've been told by a plumber that doing this may drop the mains
pressure into my flat, so won't necessarily solve the problem.

Actually, it will solve the initial problem.
Your problem is that you are trying to mix mains pressure and tank pressure.
This is usually not a good idea.
If you have mains pressure cold to your shower, then the mixer will work
fine, because the Combi is also at mains pressure.
You should still get a reasonable flow, because you will be drawing less hot
water once you mix in the cold.
Only problem is if the mains pressure is so low that it is below spec. for
the combi when a mains cold tap anywhere in the flat is turned on. You do
have mains cold in at least the kitchen, don't you, for drinking water? Not
a good idea to drink water from a storage tank! Think dead mouse.

Only other thing I can think of at the moment is to install your own cold
water storage tank, plus a booster pump, to give your own dedicated tank fed
(boosted) cold supply to the shower. However this seems like overkill.

HTH
Dave R