Genuine Dudley Hydroflo replacement rubber diaphragm washer compatible with all Hydroflo inlet float valves such as these. (opens in new window). Part number PSPBVM319726
This inlet valve washer is 28mm diameter and 8mm deep.
If the toilet cistern is overflowing or not filling replacement of this rubber washer is a cheap and simple DIY task once the water has been turned off.