I made one final tweak before going out... I hadn't checked the cleats on my shoes, then I remembered why - the hexagonal holes in 3 of the 4 bolt heads are not hexagonal anymore! At some point in the future, I may have to do some serious bodging to get the old bolts out. Nowt to do with them for now though. I did manage to loosen the 4th bolt, and guess what...? Yes, it was corroded and creaked as I undid it! I lubed the thread and then tightened and loosened the bolt a few times, before doing it up as tight as I could without risking damaging that one too.
While I was at it I tried to get some oil under the cleats and also put a thin coat on top of them. If the cleats had been making any noise, that was about all I could do for now to reduce it.
I didn't end up doing a long ride, but 5 km including some short 10% ramps was enough to tell me what I had been dreading finding out...
... Yes - I've finally run out of noises to complain about!
I'm sure that something will go wrong again soon, but for now the bike is beautifully quiet and smooth to ride. The only things left to moan about are the local traffic, the local road surfaces, and the unfitness of the bike's designated rider!