hi dave have you tried it with the shower head removed ie water out the hose only ? shower head may be blocked and need stripping and cleaning it maybe restricting the water flow this will cause low pressure to activate. always start where the water exits and work your way back .next step would be try it without the shower head and hose and so on .do you see where we are heading its useually a blockage .mainly limescale.
richard chapman, May 2010