Thank you pert for the solution.
I upgraded from IDE 1.8.5 to 1.8.7 a few days ago, but it was only today when I wanted to amend an old Nano program that I fell down this pothole. Fortunately after a very frustrating hour or two I ended up here.
Funny really. A few days ago I posted a topic The need for speedy access to known solutions and a plea for full revision histories of software libraries. pert was one of the few people who could see what I was getting at.
Of course the answer to this Nano problem is hidden away in 12,500 words of Arduino Software Release Notes. But who would think of looking there?
This is one of the annoying aspects of the Arduino. Although it is great fun, it is so 1960's - when weekly updates used to arrive highlighting the effect of changes that had been made weeks or months before.
Somewhere someone must have known that this update was going to affect people with Nanos, but without speedy access to known solutions a whole crowd of people (looks like more than 26,000!!!) have been floundering around. I rest my case.