After the major revamp, I took the bike for a test run, out to Amersham and back, doing several caches along the way. There were three problems.
The first was that the front wheel was going bump ... bump ... bump as I went along. I knew what the problem was - the tire was worn on the sides. It's because the bike gets clogged up with mud, and this rubs against the sidewalls and wears them. Looking at it carefully, I could see that the wear was pretty bad, and I don't want a blow-out when I'm mikes from anywhere. So I replaced the tire, which wasn't a big job.
The second problem was a grinding noise as I pedalled. I tracked that down to one of the pedals. I gave it a shot of oil, and now it runs smoothly; I also put a rubber cap on it to stop water and mud getting in. I might have to replace that pedal eventually.
The third problem was the bike getting stuck in the highest gear, but that seems to be an intermittent problem, and I'll go with it; I'm planning to go out tomorrow.
Something useful I learned - plastic tire levers aren't a good idea; when you put a lot of leverage on it, it snaps.