Gainsborough 9.5sdl Electric shower pressure problem?

Have had 2 years service from our Gainsborough shower however there is a problem with the pressure, upon turning the shower on we get a trickle of water coming out, if you open the bath tap to full the shower springs to life.
I have take the shower head off and also run the shower with the hose off into a bucket and still no change ??

Any ideas on fixing it ??

Steve