Underfloor Heating Mixer ?

[mikewill]

Hi

I have one of these and was hoping to adjust the flow
Not sure how it works, but the top bit has a screwdriver slot which you cant turn much (even if you undo the nut below it)

And when the nut is loosened, water drips out of it

Was hoping to open it to the max setting - the make is FAR

Any ideas

Thanks