Plumbing problem - running toilet

[I_M_Weasel]

I've developed a toilet problem - after flush the float is rising and pushing on the handle on the valve but nothing happens - water continues to enter the toilet and go over the overflow.

However after sitting like this for 5 minutes or so the valve makes a kind of water hissing noise - followed by a large water flow that adds a large volume of water into the cistern and this ends the flush cycle.

Part in question is:

https://shop.kingswayplumbing.co.uk/wirquin-jollyfill-side-entry-inlet-valve-1737-p.asp

My understanding was that essentially the incoming water lifts the float which turns off the incoming water - what is this surge of water that happens at the end of the cycle?

Thanks for any advice

 

[Thebug]

Replace the diaphram, its in the link you posted, it normally splits

 

[kinetic747]

Replace the diaphram, its in the link you posted, it normally splits

Replaced a few similar diaphragms in our toilet, I think they dissolve in water lol.