Lack of Shower Pressure, everything else ok?

Hi there,

I have a Jive Cascade thermostatic shower with a Bristan valve that has a Stuart Turner pump behind it. Since yesterday the shower starts of delivering pressure and then weakens off to a trickle. All other water supplies are ok (bath, sink, toilet and kitchen). It is just the shower that will not supply any feed.

I removed the valve yesterday from the shower mixer unit and turned the water back on but the same thing happens (water dribbling out). I have tried it with the shower pump turned on and off but with no effect.

I have run out of ideas so if anyone can help I would really appreciate it. The only other thing I could think of would be to remove the whole unit from the wall to see if the feed to the mixing valve is faulty or if it is the mixing valve itself that is causing an issue.

PLEASE HELP!!!!!!!! :)