While I was waiting for the replacement crank to arrive I did a bit of research on-line. I found some photos of the same shimano 105 crank arm that I have fitted to the Alice and discovered that it wasn’t stripped after all. The arm’s ribbed section is only on the inner half and the outer section is supposed to be smooth. I’m not sure why it is manufactured like this as it would seem to offer a better grip if the ridges were the complete width of the fitting. As it slips straight on I can’t work out a reason for this.

Anyway, the crank arm seems secure enough. It would appear that I was correct in thinking that I had just not tightened it up securely enough the last time I put it together. Luckily (for once) Amazon had lost the package I ordered and the supplier was good enough to refund my money. So apart from the time I wasted there was no harm done and I learned a worthwhile lesson.