As I’d feared, the new pad I ordered, via Amazon, wasn’t an accurate match; the four mounting holes on my sander being just a wee bit closer together. This can be seen in the pic’ below (orbital sander ring, on top of the new pad, illustrates the issue).
Thinking how to fix things…
In the end I used the holes on the little metal ring (that goes ‘twixt pad and sander) to drill more accurately located holes on the new pad. To my great relief, this worked.
To get accurately drilled holes in the hard plastic (and thence through soft foam!), I double/clamped the pad to some sacrificial timber.
It seems that a good few of these generic near brand-less power tools suffer from similar issues, re maintenance: non-standard fittings, and non-existent support!
But if I can muddle through, and keep them functioning acceptably? All told, at the end o’ the day, it seems to have worked. I now have a working orbital sander again.