Bristan Thermostatic Cartridge Valve?

Hi - Can anyone advise what may be causing a Bristan Cart 5 (067322) thermostic valve assembly to suddenly only allow a tepid flow of water and if repair is a practical DIY proposition please?

This valve is less than 12 months old but was supplied by Bristan under the 5 year warranty that lapsed a few months ago.

This is the third cartridge I've replaced in as many years (two others through siezure).

Thanks in advance of any advice anyone is able to offer

Dean