This is a simple step by step guide to servicing the cartridge used in the Bristan 1850 shower series. This will show you how to remove the cartridge from the valve body, break the cartridge down and replace the seals. This particular video shows the 1850-2 cartridge, but the principles are generic across the range.
You can find the cartridge for this product here.
You can find the seal kit for this product here.
You can find the thermostat assembly here.
You can find the piston assembly here.