I think I have sorted this, it is called an oblong hole punch, it is made for punching holes in car body panels and is used instead of drilling prior to mig brazing..
If your not mig brazing ,you would be better off with a joggler ,they have a hole punch combined normally and is adequate for mig weldeing £25-35 fom machine mart and other places........Gav
These 'Slotters' cost around £245.00, (unless somebody knows where to get them cheaper) I offered him a Joggler/Punch but he insisted on a Slotter, apparently he does a lot ot Mig Brazing.