how can i stop my mira 722 shower leaking?

hi there, i have a mira 722 shower which has been leaking for almost 7 years now.. (never got round to fixing it).. it still works fine but the control handle drips. its a very slow drip when it is off but when you turn the handle, even slightly the drip becomes a trickle.. I just wanted to know if it is repairable or if its knackered..
Thank you for any help you can offer.