I think i may be narrowing this down after studing your post Darren.
All modules have 01312 fault code stored, relating to powertrain canbus.
They are all connected indirectly via can gateway, therefore the airbag module as that particular fault code stored, i ask myself how is it stored there,either the powertrain or steering is not communicating(as they are on the same bus), or the gateway is not passing the info onwards-or wiring between powertrain steering and gateway ?. I may be way out here, but for me it makes sense.