While there were a lot of errors about half said "ECU not recognised", but the others did not. Of these, I did NOT try to clear down error codes relating to Diesel additive (as it requested engine type), or the ones relating to the RH/LH reversing reels (there was an unlock request). I was able to clear down those that did not say "ECU not recognised" (I stayed well clear of any section which had this message (even if there were other errors in the section), as I did not want to brick anything), ran the diagnostic again (those errors had not returned), and then we went for a short run (about a mile) and tried again (these errors remained cleared). This was done through DiagBox, not Lexia.
We looked at all the error messages (even those in sections with an "ECU not recognised" message), and Matt took photos of each message (the summary part, as some had a LOT of details).
The airbag system threw up two errors, and one of these was "ECU not recognised", so I did not do anything here in the menus. However, Matt was looking underneath the driver seat (from behind) to see where the wiring was, and I pointed out it was at the front. The ignition was on, and when I touched the wiring bundle for the nearside front seat a warning message then popped up on the instrument display, relating to the airbag/pretensioner. Because of this I feel fairly confident we have a wiring issue; either a weak wire or a bad connection. I have suggested that (after following the correct disconnect procedure and then waiting for at least 10 minutes) it could be worth disconnecting the seat plugs, spraying in some PROPER contact cleaner, plug/unplug the connectors a few times (finally leaving them connected), reconnect the battery and see what messages come up.