Ah - thanks WW - but mine hasn't got a locking nut! No nut; no grub screws; no circlip. No visible locking mechanism of any kind: and it can't be simply screwed on tight and held through friction, because, as I say, it's now loose - but only for 15 degrees or so either way. It'll also move up and down in relation to the stick - a couple of mm.
