After both bolts which hold the exhaust plate on completely sheared, and after drilling out what was left of the studs, and then trying to tap a new thread into the remains of the metalwork...
I discovered a jubliee clip with a protruding 5mm stud welded on, and the rest was plain sailing.
I guess if you drilled a jubilee clip and inserted a standard 5mm bolt from the inside (ie with the bolt head against the exhaust pipe) this would achieve the same thing.
Anyway fixed each of them half inch to the right of the existing brackets and the cover plate fitted straight back on.
Not so much a bodge as a handy tip!