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

Dean
March 2010