replace it for new as they do tend to suffer corrosion problems.
That is precisely what I intend(ed) to do after a few cold morning no crank issues.
Good news, new starter earth cable installed. Access to starter end of the cable is probably best done from below.
Bad news, no crank with key issue was
not resolved with newly fitted starter earth cable.
Good news, I think I found what the actual problem was (see video below).
Bad news, will probably have to live with it, for now at least.
It's probably not obvious from the video, but on the second attempt, I manipulated the key in the ignition barrel and it fired up straight away. On the other two attempts, the ignition barrel was weighed down by the weight of the inserted key.