I understood the old float ball valve type with the slider that carried a washer against the nozzle, and the newer variations on that which used a diaphragm, but this flat has some modern contraption and I'm having trouble understanding how it works.
Close coupled, bottom entry, float operates a lever on the top that closes an air hole that I believe leads to water flowing through a hole in the middle of the diaphragm then forcing the diaphragm closed on the basis that the water pressure is acting on a larger area on one side of the diaphragm than the other.
The hole in the middle of the diaphragm has a small plastic insert thingy in it which I think is meant to be a one way valve.
The float sits in a cup that drains through a one way flap when the cistern empties so that as the cistern fills, the float doesn't rise until the water level reaches the rim of the cup, I believe this is so that the cistern refills quickly until it reaches the stop point, as the float valve only moves when the cistern is almost full. This cup is part of the inlet tube, and the float surrounds the inlet tube.
Can't find anything on the web that describes this sort of valve operation, which means I'm guessing as to how it actually works and unsure about how to try and fix it. As far as I can tell there are two rubber parts, the diaphragm with the plastic thingy that might be a one- way valve in the middle, and the little bung that gets pressed down on the top hole as the float rises.
Damn thing leaks for about 20 - 40 minutes after flushing before it settles down. I've had it all apart once and made sure nothing appears to be scaled up, diaphragm not split etc.
I'm unable to see any indication of the manufacturer on the device. There are similar looking items in the screwfix catalogue, but nothing that looks the same.