A: If the original piston in your shower was metal then you have the choice of refitting it with your thermostat or buying a new thermostat to fit the new plastic piston.
Also check that you have the thermostat in correctly, it should have the small dia steel pin pointing towards you as you stand in front of the shower.
You may also need to adjust the temp range by removing handle retaining scre ( water will come out the hole, this is normal) inserting small thin flat bladed screwdriver engaging adjusting plunger and turning clock wise to decrease and vice versa, after turning it takes a few seconds for temp to stabilise so allow for this when adjusting.
If you click on the link below it will take you to an exploded diagram and all online ordering if required.
http://www.showerdoc.com/shower-spares/sirrus-by-gummers/GUM-PARENT-34-Sirrus-1500---Thermostatic-Shower Answered 553 days ago | Alastair