Paul
I still having problems with my boiler.
I'm not sure how to isolate the supply as you advised below.
It isn't just the shower it also effects the taps (not mixers) in the bathroom.
The pressure is always going below green and I have to top it up every few days - more if I have...
I have an Alpha CB28 boiler. The water in the shower keeps going hot and cold. ie 1 min to get hot | hot for 30 seconds | cold for 45 seconds | hot for 30 seconds....
The engineer that installed it is not sure what to do.
Any ideas?