Having tested on a second DSS system that we have I can clarify the exact issue.

Once you have uploaded a software update (pressed the green tick box), the
MAINTENANCE->software update section changes to allow you to either
reboot from the new update or the old version.

My point is that at this stage there is no visibility of the new version number,
and you can't go back and re-upload a software update.
So, in my case I must have uploaded an update some time ago, but I can't
tell what version it is, or upload the latest version instead.

Not a major problem, but I would recommend that the software update
section clearly indicates which version is about to be rebooted to, and
also offers a function to upload a different update at this stage.