You will need to clean the shower filters.. these are located within the shower unit. I have performed this a few times quite successfully... do the following:
- turn off all power to the shower, probably best to trip the switch in the fuse box
- Open the 4 screws (two on the top and two on the bottom)
- Remove the cover and also the face plate that shields the piping.
- Using a spanner/vice grip open the nut that is pointing outwards/diagonally
- when this is open full it will result in water gushing out.. all over the palace. take the nut which has the filter attached and clean it
- Replace the nut and dry up all the wet areas.
- Replace all the above... again drying up the wet areas.
- Turn on the power again... have a nice shower
Note: Care and attention is needed while doing this, self explanatory...
All the information to access the filter id documented in the user manual for the shower... if you still have it.