Thanks so much chaps. I had a closer look and it's a split pin thing by the looks of it. The problem was not so much not having a corner steady, as having one that was hanging off :-( I've managed to put it back together after realising taking it off altogether was too difficult. The pin seems to have sheared off, so if I unwind/wind up the corner steady too far, it comes apart. I don't think I can fix this myself as the broken-off pin is still in the middle of the rod, so would need pushing/drilling out with something in order to fit a new one. I guess this is one of the 'joys' of an old Puck, but annoying as I was assured everything was in good order when I bought it.