asuming that you have turned both the shower and the isolating switch off and that the water coming out is a slow flow, not pressured it sould be just dirt in the valve mechanism.Mira is a brill shower ,good normally for 10-12 years, 3 things to do, both switches off, take off cover, where water pipe comes into shower you will see a brass nut about the size of 50 cents , put plastic bag securely under nut to stop water falling onto shower, loosen nut, water will come out , be prepared, take nut out fully, water will flow rapidly, close door as much as you can, you will see a metal filter , clean same, put back, your valve mechanism will be the one where you turn the water from cold to hot, turn to cold gently tap with the back of a screwdriver or similar, may loosen dirt, if that doesnt work next step is dangerous for some, and i stress that, it requires all switches on shower turned full cold again tap gently, if dirt loosens shwer will flow full, CONNECTION WILL BE LIVE.