Well named

That little screw in the middle is called the "b" screw.
Gear changes on the bike were imprecise and noisy and I suspected this little blighter.
It was partly to blame, main problem was the gear hanger was fouling the screw and tilting the mechanism.
A bit of careful filling, reference to the fitting instructions and it now seems to work properly. Not been out on the road though - it's raining again.