You can get a pack of seals.
http://www.showerdoc.com/MIRA-441-27-Mira-Seal-Pack-Fittings---Components-Identified-‘c’-441-27
However if you have a temperature or dripping problem then unfortunately you are going to need a new cartridge.
http://www.showerdoc.com/MEYN-SPSL0006J-Meynell-Sleeve-Cartridge-Assembly-(0-Onwards)-SPSL0006J