Depends on exact model, but normally you'd be looking at buying and replacing the cartridge within the shower valve body. A search online for Mira Excel Cartridges will help you identify the correct part. Not a hard job to do as long as you can isolate the hot and cold water supplies.
It will probably look like this:
https://www.showerdoc.com/mira-excel-1990-96-cartridge-903-33?channable=e79264.Mjg1ODk&gclid=EAIaIQobChMI9dfj2PK65gIVmpntCh3NqAU7EAQYASABEgLAW_D_BwE
Rupert
December 2019
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