Problem with chain `skipping` (but not trying to shift)
It turns out that it was a stiff link, stripping the chain off the bike, dousing it in brake cleaner, then oil and leaving it to bathe a few minutes, then repeating the process left the chain rust and dirt free, looking like new.

When I took off the chain, I broke it at the stuck link (which was clearly evident going through the derailleur when pedalling the bike backwards) -- cleaned everythingh whilst I had the opportunity and reassembled.

Whilst the chain was off, I did measure it, and things were within tolerance, although (I assume naturally) a little stretched.

Thanks for the help, I now have again a reliable bike!