First check that all the valves to the boiler and the showerare fully open, because I suspect you may have a flow rate problem, if all ofthese are OK and you are happy that the boiler is OK then that only leaves afaulty shower cartridge.
http://www.showerdoc.com/shower-spares/mira/MIRA-902-55-Mira-Cartridge-Assembly-Black-902-55
It is almost certainly a faulty cartridge, unfortunately the cartridge is now obsolete.
Probably the best option is to replace it with a Mira Combiforce 415 this is not a direct changeover however it is probably the best option.
http://www.showerdoc.com/ZMIRA-1-1542-001-MIRA-COMBIFORCE-415-EV-CHROME
The knob just pulls off at the front and I am as near certain as I can be that the front cover just unclips if i remember correctly you just need to use a up and down rocking motion
This is a pressure balancing valve most of the Aqualisa valves can now be used in place of this valve, however if you need pressure balancing valve probably best to go for the Mira 425 or the combiforce.