Replacing cartridge in Bristan thermostatic shower mixer?

After I replaced the cartridge into the housing I can't turn off the flow of water. Can anyone explain about the ceramic valve refered to in servicing instructions (I can't see anything resembling this on mine) and the correct orientation if any, of the cartridge to slide into housing. Have new seals on order but need to sort this out first as it worked OK before dismantling it.