Hi Richard,
It appears that UpdraftPlus is taking a long time to process all the files in the WP Core backup, which is considerably larger than the rest of the site.
From the log, it appears that there may be some directories being backed up that are not required. These may reduce the size of the backup if you can exclude them:
If any of these contain separate WordPress installations, or any un-needed files, I would recommend adding them to the WP Core exclusion list.
Best Wishes,
David