Apologies to both user2640725 and Joseph for the lack of feedback to your suggestions and to thank you for taking the time to respond to my question. I assumed I would be notified of answers to my question as a default - I have since changed this in my preferences
It seems that the problem was down to the "A" option in
rsync -aqv -HAX --delete $BACKUP_SRC $BACKUP_DST$DAY
According to Rsync Man pages
-A, --acls preserve ACLs (implies -p)
As soon as this was removed - the error went away. What is strange is that while testing this - I have the source and destination folders in the same file system - so I do not know what why this is causing a problem.
Will test a little more to see if there are some strange side effects to removing this option - but for the time being I prefer at least to have a copy of the data if at the expense of ACL attributes not being carried over.