A: First of all check that hose is not kinked or internally collapsed and that sprayhead is clean and there has been no reduction in mains water supply to property.
If all of these ok then it is the pilot valve, which can be ordered online by clicking on the link below.
http://www.showerdoc.com/shower-spares/mira/MIRA-419-87-Pilot-ValveAnswered 28 days ago | Alastair