Restore - Backup List. How to remove entries?

I generally keep 2 generations of a disk backup so after creating a third I manually delete the folder on the server containing the oldest backup.

However, this doesn't appear to remove the entry from Restore - Backup List. There are entries there going back a few years but the actual backup doesn't exist.

Could you please explain how to delete these entries and how old backups should be removed so the Backup List is accurate?

Thanks.