mira sport shower?

A couple of solutions...
1. Clean the shower head to get rid of lime scale. This simple step might be enough.
2. If that fails turn off the power, turn off the water take off the plastic cover, where the water comes in there is black nut, unscrew and clean out the filter.

Here is how the shower looks inside http://www.onlineplumbingsupplies.co.uk/acatalog/Mira_Sport_7_5___9_0Kw___1998___Present.html, you are looking to clean "406.27 - Inlet filter cap"

slice
October 2008