Cistern fills slowly?

Our cistern has started to fill up very slowly(takes apporx 8-10mins).I have looked at the inlet where the balcock is located,and the inlet of water seems to be under fairly high pressure,but is only coming through a small hole in the plastic valve..I have removed the valve and cannot work out how it works at all!!
Is the mall hole some kind of "default" hole that still allows water even if something has gone faulty????

If it helps it a cistern with a push button on top!!

Any help appreciated

Gary
December 2007