The white silocone washer on the left. Or the complete valve.
Look on fixthebog.co.uk for the same valve.
If the existing washer has pimples, pop them with a pin and squash the water out until flat.
The Fluidmaster PRO range is the preferred choice of the professional plumber The PRO820UK is a perfect, well priced replacement valve and will fit most top flush cisterns with 2″
plumbinbits.co.uk