Bristan Thermostatic cartridge Cart 5 dripping?

Hi. I've replaced the seals on this cartridge and still have a slight leak. I've fitted it according to the instructions and therefore think there may be another problem. How could I tell if the ceramic valve has failed? Basically I would like to know if I should buy a valve or a whole new cartridge. I'm no plumber -if it's a valve replacement is this practical for a layman?I would have thought if the seals are OK it would stop any seepage from a faulty valve. As you can gather I'm not very knowledgible on these matters.Any help would be gratefully received. Thanks.