Our toilet recently stopped working properly. Specifically, it was taking 10-15 minutes for the cistern to refill with water after being flushed.
The situation could not be tolerated. I had a fiddle to see if I could work out with my own two eyeballs what was up, before turning to my old friend The Internet for assistance. I wasn't disappointed. Upon searching the make of the refill mechanism, I found a forum thread discussing the exact problem.
It seemed to be the case the diaphragm might be split. This rubber washer is housed inside the ridged inlet valve (see close up below).
After turning off the mains water I unscrewed the valve and examined the diaphragm. And what do you know, there was a tiny tear in the rubber. Although only a couple of millimetres, this hole totally ruined the pressure-operated mechanism.
I quickly fitted the new part (above left) and found the flush to be as good as new.