Lever flush cistern parts list

[denso13]

So what you are saying the clamping plate does not compress the doughnut

Yes, it compresses the washer onto the pan

 

[dilalio]

The rubber gasket that goes onto the tailpipe of the flush valve, before it is pushed through the cistern outlet, creates a watertight seal between the flush valve and the cistern, when the plate and backnut is connected and secured tightly - to stop water leaking out of the cistern when it's full.
The donut washer stops water leaking out between the cistern and the pan "during a flush".

 

[johnscts]

Many thanks for the link just looked at the link and it looks as though the plate screws directly on
To the cistern ceramic base ( no nylon washer) I have to say thank you for all your patience and help. I am sure in the very distant past a clamped the doughnut to the plate which was a mistake. Looking at the YouTube link the place your man replaced was well corroded ( no Vaseline or silicon grease used previously. ps I don’t quite know how I add to your points but your help was 10/10
Kind regards john