Could be a few things however the first thing I would checkis the inlet filters to make sure that they are clear
To fix the temperature problem first check that the filtersare clear.
http://www.showerdoc.com/shower-spares/mira/MIRA-PARENT-151-Mira-Essentials-Manual-Power-Shower---post-1999
If the on off is sticking then you really need to replacethe valve.
http://www.showerdoc.com/shower-spares/mira/MIRA-209-70-Mira-Manual-Valve-Assy-209-70
However with a little care this valve can be stripes lubedwith silicon grease and rebuilt
http://www.showerdoc.com/tools-and-fittings/plumbing-consumables/MCSILICONE-Silicone-Grease-Tub-30ml
To solve this problem you really need to fit a new flow control valve
http://www.showerdoc.com/MIRA-209-70-Mira-Manual-Valve-Assy-209-70
However you may get away with fitting a new bucket seal you can see the end spring of this sticking out of the flow control valve.
This seal can be tricky to fit however it is a lot cheaper than the flow control valve.