Finally resolved the diminishing lack of free disk space for future FreeBSD updates. It seems that since version xx.x each upgrade as made a ZFS snapshot taking 1-3Gb each for each successive version. As the hardware has been running since 2017 that’s an awful lot of major and point revision snapshots.
DU and DF hid those successive snapshots and I was blaming my ever expanding OneDrive offline sync for taking excessive amounts of disk space even though I was convinced I had move the sync folder to the slave disk some time ago.
As I have never needed to roll back a ZFS snapshot I had never needed to explore what amount of space they took, or even how to display their usage, even less so how to delete them. Necessity being the mother of all invention, or least the need for a proper Google session I finally found the commands and confidence to delete 12 previous incarnations of FreeBSD and give me back the free disk I need to apply the next edition of FreeBSD.