So I added the magic incantation
dwc_otg.speed=1into the configuration file /boot/cmdline.txt and things are looking a lot better.
The up time is over four days at the time of writing, which is a lot better than it achieved before and I am currently letting Polly-Pi take over live control periodically, when a crash wouldn't be too inconvenient.
Now it's a bit early to be getting excited, all the previous incarnations of Polly have a total score of one crash in almost twenty years of continuous running, so four days hardly proves anything, but we'll see . . .
Update 09 April: Still going strong. . .