Thanks for the 'Leo' link. I tried a masonry bit but I didn't think to regrind it – and it just polished the bottom odf the hole. I've had one work in the past on a wheel stud,
I'm afraid I have done a disgusting bodge and turned each of the mounting holes into a slot with the angle grinder. the new part has holes on a triangle and the slots are such the screws won't be able to slip out. It ain't clever and it ain't pretty. But it's prettier than this example I found on t'web:
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The god's have punished me with a stiff neck so I can hardly look up, down left or right! At least the dodgy handbrake (brake adjuster had slipped off the ratchet) was easy enough and remaining job is a brake pip, that for once appears to be wholly accessible once the ramps are at the back.
Neil