Driving home felt something knocking. Got home and discovered that the worn ARB bush had left enough play for the bar to knock the mount about enough for a bolt head to shear.
Now held in place with just one stud.
If it could have waited until the weekend, I would have been replacing both the bushes anyway.
Now I have to go see the garage I use first thing, phone work tell them I'm going to be late, and see if he will drill and retap the hole and get me another bolt in.
He's usually pretty good in an emergency.