Here is my solution for my Gravity feed Aqualisa electric shower.
Problem;- The Water board cut off water supply for several hours. Water supply was then restored. All taps working but there was only a dribble from the shower head.
Solution. Remove the shower spray head. Switch the shower ON. Push the shower hose, with the rubber washer, hard up ( very hard so there is no leak ! ) against the cold tap in the bath ( or basin ). Turn the cold water tap full on for 15 seconds.
Turn off, and reconnect the shower head. Then the shower should work again. In my case it did - brilliant result ! Good luck !
Bundoo
November 2011