SORTED!

For years I have used Acronis backup, which allowed me to backup to a local drive and online, and I'd do full backups weekly to the drive, incremental nightly and nightly online backups of things like photos.

The online came with an annual renewal charge and because I was in the midst of moving house and other things I let it lapse for a year or two, but recently decided to reinstate it. Like so many other companies they had completely revamped it into something that was almost unusable so in the end I switched to MyPCBackup. (In many ways that's inferior, because you can only do online, and trying to back up a full system is not practical with a few Gb/s upload.)

Anyway, searching around I discover that Acronis use a version of 'snapman' which can cause problems for Windows 7, so I removed all traces of Acronis and my drives have reappeared.