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:
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix baths and showers.