Turn off the water remove the knob (8) temp limit stop (9) then unscrew the chrome shroud (10) and then just pull the cartridge out.
http://www.showerdoc.com/shower-spares/mira/MIRA-PARENT-28-Mira-Coda
Sounds like you need a new flow cartridge.
http://showerdoc.com/shower-spares/MIRA-1663-117-Flow-Cartridge