How do I speed up the fill-rate of my push button W.C cistern?

I have a small modern push-button toilet cistern in my recently converted shower room. It has performed well for the last four or five years, but has now started filling very slowly, and more noisily than before. The stop-cock cosists of a slim, compact nylon float chamber, which rises vertically (rather than swinging up on the end of a lever like the old-fahioned copper ball-cock systems that I've seen before) as the water level rises. It seems to open and close the inlet valve just fine, but the water is released as a hissing jet rather than a smoothly flowing stream. I can't find any way to alter the flow rate of this valve. Any ideas?