Why does my toilet keeps filling and flushing over and over- Help please.

[PSH]

Our toilet cistern has a two year old Armitage Shanks 8.5 “ bottom entry fill valve which will not shut off the water when refilling after a flush, but just keeps flushing. At the bottom of the tank is a small amount of silt or brown powdery dirt , which made me think the diaphragm inside the unit may be clogged, so i stripped it down and everything looks fine . After I had put it back together ,it did shut off the water once the small blue float pushed on the lever at the top of the unit ,but then on the second flush it kept filling and flushing over and over ,then stopped after five minutes .What can be causing this ,do I need to renew the black diaphragm, which has a very faint circular indentation on its mating surface, or should i fit a complete new unit, say a Fluidmaster or go for another new Armitage Shanks version?.
Is there any where in the Coventry/Midlands area that would stock a new on/off valve ,which is an Ideal Standard s900167 iso valve plastic outlet valve, Product Code: B78944.

Thanks
Paul.

 

[lame plumber]

diaphragm has dirt in it or hardened replace it with identical one or just fit a new fluid master.

 

[PSH]

There was no dirt when i took it apart,and could not find any where that did replacement diaphrams,so i got a new armitage shanks unit from Screwfix,and all is fine now.

thanks Lame plumber.
Paul