Looking for siamp toilet part

[Luckyprincessuk]

Hi, I'm looking for a siamp part, it's the white washer under the cap that flips off.
I've recently changed the inlet valve and realised the water isn't cutting off because this washer has become hard and brittle so it's not moving.
I can't seem to find this part. btw, I don't know what's it's called because siamps diagrams are unhelpful. Any help will be appreciated.

 

[snowhead]

Wrong part,, back soon,,,,

 

[snowhead]

This one?

Toilet and Cistern Spare Parts -Kingsway Plumbing

Large selection of toilet and cistern spare parts available from stock for quick delivery.

shop.kingswayplumbing.co.uk

 

[Luckyprincessuk]

The part under this cap, it's like a diaphragm that stops the water flow when the float raises.