Hi

Here is the link to the installation instructions for the Mira 88 Gem (the 88 replacement)

Depending on how your existing unit is piped I think you would have to cut into the wall to replace your shower.

http://www.mirashowers.co.uk/onlinecatalog/pdf/1061361_w2_a.pdf

The other possibility is to replace the internals of your existing shower, by fitting a new internal set you are replacing virtually all the working parts.

You can also consider fitting a new knob set which will just make it look newer.

http://www.showerdoc.com/shower-spares/mira/MIRA-PARENT-76-Mira-88-B---post-1984

http://www.showerdoc.com/shower-spares/mira/MIRA-904-07-Internal-Set

http://www.showerdoc.com/shower-spares/mira/MIRA-937-84-Knob-Pack-Components-Identified-C-(88-Exposed--Built-In)