So my QNAP TS-251B is fixed. Not quite sure how tbh. I read a write-up that walked you through a reset but advised that it would wipe the drives. Luckily, I could still access the NAS from the cloud so downloaded all the important stuff. anyway, I bit the bullet and pressed reset and the voice said "Advanced Reset started" after a reboot, the NAS was back and so was my content
Had to re-implement my personal configuration such as fixed IP etc but well chuffed it didn't wipe the drives.