As per usual, I fix one problem and another appears.
This time the airbag warning light came on for a moment, then stayed on, then was off the following day but came on again momentarily.
I read the codes and I got two errors:
1) Ignition circuit 2 - passenger tensioner: remain over limiting value
2) Fault lamp: open circuit or short circuit to ground
Time to investigate the common causes of these errors.
I did some online research and found some ways to bypass the passenger tensioner by fitting a resistor.
I also found a few posts where people had found that this error was caused by a bad connection.
I like those kind of fixes (ie. free) so crossed my fingers and looked under the seat.
All of the connections seemed to be okay but I pulled them apart and pushed them back together again and cleared the fault codes.
A month later and I have not had the light come on again, so if you have this error it's worth checking the connectors under the passenger seat.