OK, given the silence, it appears that restoring a backup DOES rely on your browser's web cache of a page that no longer exists after a failed upgrade. This is terrible design and the page should contain a warning: CAUTION: DO NOT NAVIGATE AWAY FROM THIS PAGE IF YOU WISH TO RESTORE YOUR SHOP!
I now have 147 database backup files named :
auto-backupdb_000001_V1.5.6.0_20190304-221520-41b7435c.sql.bz2
auto-backupdb_000002_V1.5.6.0_20190304-221520-41b7435c.sql.bz2
etc. Does anyone know how to use these to restore my db?