I fixed just such a problem yesterday. My shower has a tiny filter screen on the inlet pipe to the shower. It fits inside the pipe and looks like a bronze wire-mesh dome. It was clogged up with a white gunge. I had to take the shower off the wall to get at the filter. I could remove a small section ...
My initial problem was with a large downstairs radiator that does not get hot. It's the furthest from the boiler and the other 7 rads on the system are fine. I decided to drain and flush the system (I have 8mm piping) and took the offending radiator off to flush it out. It was mucky but not that bad...
