Just had a look in my toolbox. I might have started at 8 and gone down. Not very helpful sorry. It's trial and error really but it sounds like you've explored the alternatives.
Thanks for everyone for contributing their thoughts...
I rang Autolusso South, who are based near me at Ferndown, Bournemouth. The guy I spoke to was very helpful and suggested I take the vehicle to them for a quick check as they regularly resolve airbag problems on these models. As there were no ACU faults, the mechanic did not think that the ACU was causing the trouble. Despite all my un-clipping, cleaning and wire tying the connectors, I never would have resolved the issue, and here is why:
A previous owner had obviously tried to resolve this matter by taking apart the passenger side underseat connectors including cleaning the pins. However, the pins had been put back in the wrong slots and did not line up with the seat side of the loom connections. As soon as the mechanic swapped the pins round, the airbag light went out on reset. Unfortunately, although he managed to resolve the light issue, the light came back on because the connector then revealed a high resistance. Fortunately, the guy had a spare section of wiring loom complete with connectors, which I bought for a fiver. When I got home, I stripped the ends of the substitute wiring and check the resistance across the connector. My ohm meter immediately registered 'Zero' indicating good continuity across the terminals. So far so good.
I am now going to undo the seat bolts and tip back the seat to facilitate access to the wiring, then solder in the substitute connector to the existing loom.
In all, the garage charged me £35 to diagnose and change the pins around in the passenger side underseat connector, and this included the spare bit of wiring and connector. It is a shame that the existing connector was also faulty.