There is no service kit for this valve there is the thermostatic cartridge if you have temperature problems.
http://www.showerdoc.com/shower-spares/triton/TRITO-S06260700-Triton-Thermostatic-Cartridge-S06260700
Or the hot and cold flow controls if you have on/off problems.
http://www.showerdoc.com/shower-spares/triton/TRITO-7122613-Triton-Flow-Control---Cold-7122613
http://www.showerdoc.com/shower-spares/triton/TRITO-7122612-Triton-Flow-Control---Hot-(lp)-7122612
Triton don’t appear to do an ‘O’ ring kit for this valve.