Repairing the push plough

I should have been out doing this from the start of today, but it wasn’t to be. When I was cutting furrows yesterday afternoon I noticed that the plough was coming adrift from the gooseneck that holds it to the main frame. When I packed up for the day, I took the plough down to the workshop, applied a little WD40 to the nut on the back of the breast, waited a few minutes, then turned the nut about a quarter turn, and snap! The square-shouldered screw sheared off and the plough fell off the gooseneck. Oops.