How do people go about repairing the upper arm - the wear under the ball and cup when the cup wears through?
Would a little bit of Araldite work?

Yes, make sure the drain hole is clear, mix a bit up and push the socket in. Excess will come out the hole.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

devcon works too
but grease the bottom of the nylon cup so it don`t stick
& can be removed easilly

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No offence intended here but--> anyone writing a book about minis 30 years ago may not have experienced such worn or stuffed-with components as we are finding these days.

You should put your heart & soul into everything you do.