A: Check that hose is not kinked or otherwise obstructed, shower head is clear and inlet filter is not blocked.
If all above ok then the Pilot Valve needs to be replaced, it can be ordered online by clicking on the link below.
http://www.showerdoc.com/shower-spares/mira/MIRA-419-87-Pilot-ValveAnswered 18 days ago | Alastair