The air bag light came on and at my request a local non BMW garage has read the fault codes and reset the error. The lamp came on again the same day, so the fault is persistent. The air bag fault codes they read were:
02 Ignition Circuit ZK1 Belt Tensioner Driver
1A Seat Occupied Recognition 1 (SBE1)
The garage said that both of these codes referred to the driver's seat, but when I unplugged the connectors and removed both front seats, it seems that only the passenger (LH) seat has an occupancy sensor. Could these codes refer to the passenger seat, or has the system logged some spurious data?
Finally, does anyone know what the likely cause of these errors is? I have inspected the wiring harnesses under the seats and they seem in good condition, even around the stress point where the flexing harness is cable tied to the anchoring point behind the crossmember.