I.e. there is a brass thing with a hexagonal head that I think I'm to turn clockwise with a spanner until it won't move, shutting off the valve.
If that's right so far, what if anything is the purpose of the screw in the middle of the brass thing? It's a bit blurry in the picture, but it seems to be the head of a screw that goes into the axis of the brass thing.
Should I attempt to loosen or tighten the screw, or remove it, or just pretend it's not there?