Thanks, that solution worked. I wanted to backup before updating WordPress in case the upgrade broke something on the site.
For the benefit of anyone with the same problem; I commented out the line calling get_main_site_id() in plugins/updraftplus/backup.php, and sure enough, was able to run a complete backup.
Then updated WordPress (and everything was okay), uncommented the line, and tested backup again, and it’s fine.
So, my issue is now resolved. ??