A quick post script in case anyone has had a similar problem. Took the offending ATD backplate to my local welders (Specialist Welding, Hanney Road, Steventon, Oxon 01235 820821) and a chap called Frank thought he could repair it. Sure enough, one week later, I returned to find he had made a new "ear", welded it into place and welded the crack in the opposite ear. This for the very reasonable sum of £20. All I had to do was dress back the welds to ensure full movement of the toggles and to cut away a little of the tufnel gear to accomodate the thickness of the weld (I thought it better than reducing the weld thickness). All now looks very good. Nice to find a company willing to accept a challenge.
Grash
Comet