jond1 Got around to trying this and it's working really well - thanks :-)
I made one quick tweak (for my purposes which may be useful to others) to exclude the WordPress core files etc:-
tar -czf "$BACKUPPATH/$SITE/$DATEFORM-$SITE.tar.gz" --exclude-from="/usr/local/bin/s3backupexclude.txt" -C "$SITESBASE/$SITE" .
And then in s3backupexclude.txt
:-
./logs
./htdocs/wp-admin
./htdocs/wp-includes
./htdocs/wp-content/index.php
./htdocs/wp-content/upgrade
./htdocs/wp-content/cache
./htdocs/wp-content/updraft
./htdocs/wp-content/ai1wm-backups
./htdocs/wp-content/*.zip
./htdocs/wp-content/*.gz
./htdocs/wp-content/plugins/index.php
./htdocs/wp-content/themes/index.php
./htdocs/wp-*.php
./htdocs/index.php
./htdocs/license.txt
./htdocs/readme.html
./htdocs/xmlrpc.php
There's also --exclude={/logs,/htdocs/wp-admin,/htdocs/wp-includes}
if you don't want an external file list.