Got to the bottom of it I think, mostly by accident rather than design, so I've marked the thread as solved. I had tried everything that had been suggested, all to no avail, so I was just playing around trying to find out exactly what was taking up so much space. As I was going through things I was making notes as I went along, so there may well be a shorter more direct route than this, but here goes: mnt/sdb2/EasyOS/sfs/EasyOS/oe/Kirkstone.
In there I found an endless ream of old sfs's, along with the spec sheets, going right back to version 4.2 (I think). So deleted most of the old ones (left a couple behind to be on the safe side). Rebooted to make sure I hadn't over done things and wrecked it, and all was ok. Space used is now down to less than 3Gb, a massive improvement on the previous 12GB.
I'm guessing this isn't supposed to happen, unless I'm barking up the wrong tree completely? But deleting all those old sfs's is the only thing I've done that's freed up the space.