Looks like the link says it all .. most of these modern small cisterns have a fittings pack included when they are supplied. The fitting are usually from an unknown source (that's why you can buy a whole toilet for £50).
They usually work for a few years then give up ... but you often can't buy spares for them, because you can't identify them.
That's why I always ditch the fitting supplied and install a globally known named brand. You can always get spares if needed at a later date ... I just stick with Fluidmaster.
Infact if you do go to the length of replacing the inlet valve I would do the flush valve at the same time ... as a pack they are cheap as chips !
http://www.wickes.co.uk/Fluidmaster-Button-CDFV+B-E-Fill-kit/p/118744?CAWELAID=120135120001173974&CAGPSPN=pla&utm_source=google&utm_medium=cpc&utm_term=&utm_campaign=Google%20Shopping%20-%20Heating%20&%20Plumbing&tmcampid=&tmad=c&tmplaceref=lzQbnSzV&tmcustom=mkwid|slzQbnSzV_dc|pcrid|43364569612|kword||match||plid|
Eve if you do separate the valve and clean it .. be careful putting the washer back and watch out for the pin.
With this valve it only takes a flake of scale to block it !