First make sure that all your valves are fully open then I would check that the inlet filters are clean, if they are OK then you are going to need a new cartridge.
http://showerdoc.com/shower-spares/mira-parent-32-mira-combiforce---exposed
http://showerdoc.com/shower-spares/MIRA-902-55-Cartridge-Assembly
I am not sure which part you mean however this is the flowcartridge
http://www.showerdoc.com/MIRA-459-05-Mira-Flow-Cartridge-Assembly-459-05
All the other parts are here
http://www.showerdoc.com/mira-parent-30-mira-combiflow---exposed
If you can’t work out which part you need then send me acouple of photos
If it is dripping through the shower head then you will need a new flow cartridge.
http://www.showerdoc.com/shower-spares/mira/MIRA-459-05-Mira-Flow-Cartridge-Assembly-459-05
I am not exactly sure what you mean by back pressure and what exactly the “back pressure problems” are, however the part in the shower valve that controls the pressure is the pressure balancing cartridge.
http://www.showerdoc.com/shower-spares/mira/MIRA-PARENT-30-Mira-Combiflow---Exposed
http://www.showerdoc.com/shower-spares/mira/MIRA-459-07-Mira-Pressure-Balancing-Cartridge-459-07
Yes here is the link
http://www.showerdoc.com/mira-459-18-mira-cover-shroud-assembly-459-18
Hi
It probably just needs a new flow cartridge here is the link.
http://www.showerdoc.com/shower-spares/mira/MIRA-459-05-Flow-Cartridge-Assembly
It is just possible that it needs a new Pressure Balancing Cartridge However I would replace the flow cartridge first.
Link to PBC
http://www.showerdoc.com/shower-spares/mira/MIRA-459-07-Pressure-Balancing-Cartridge