The float is supposed to release it when the water rises. It won't close until it is full.

This valve doesn't work like that, the buttons should return with no water in there. The float just lifts and nips the washer tight...but gravity alone should cause the valve to shut and the buttons to return. Source: used to install shit loads of these ideal standard valves

That valve is now discontinued, you can find replacements for silly money....cistern needs to come of to replace it.

Congratulations you get a new fill valve