Yes, that was it - problem solved. I created a clean new S3 bucket for the backup location and now the backup/index works quickly.
Problem Summary: With the introduction of direct backups to S3 using the new Backup Location feature, the listing of objects in S3 at the ‘S3 backup location’ setting can potentially take a long time, and cause a timeout over 30 seconds of the /admin and /admin/backup pages. This didn’t used to matter before, as the local file system was used as an interim storage location, but with a direct S3 backup the backup/index needed to list all the S3 objects it found.
Solution: Choose a S3 backup bucket with less existing objects in it, i.e. do not share your S3 Uploads location with your Backup Location S3 bucket.
Thanks for info all - all working great now.