Dual Flush toilet keeps on filing?

probably a common issue - have a dual flush toilet with a bottom fed boat float valve (like a Roca 1/2" on lunns website). The cistern keeps filing until the water pours down the central cycliner of the Flush valve (looks like a Roca D2D post 2006 on Lunns website), into the toilet bowl, so I am assuming that the float valve is not cutting off.

I have tried adjusting the float by turning the screw thread at the top, but to no avail - is there any other ways of adjusting the float valve?

In addition, there is a plastic flap that hangs down where the vertical screw rod joins the top arm - I cant see it in the picture on the Lunns website, so does it need to be clicked in anywhere?

Thanks