OK, have spoken to the Service Manager at the dealer and the official situation is this.
I would have to pay for the TECH II diagnostic, and the labour charge for fitting a new sensor, I would not be charged for the sensor.
The sensor would be sent back to Vx, and if it proved to be faulty, I would be refunded the Tech II charge and the labour charge.
He says, in all the time he has been a Service Manager he hasn't known an Omega drivers door airbag sensor to fail, and as it has failed again to fully check the wiring, and connections.
In principal this sounds to me like a fair deal - they do the work, you pay, if the part proves to be faulty they refund you for the work.
However.....
If they are sending the part back to Vx to be checked anyway, then why not take it off yourself and give it to them, ask them to send it back to Vx as they normally would, then let you know what the score is - all FOC?
Also, if nit picking, then dealers usually charge for Tech-II diagnostics if you come in for disgnostic only and do not proceed with the repair, but if you book the car in and pay for the repair then the Tech-II diags is usally done as part of the service and not charged separately. So in this case you should only be really paying for replacing the sensor, not for Tech-II..?
And last - what happens if they change the sensor and the problem is still there...? Might be a good idea to agree their remit in advance, i.e. either 'just replace the sensor', or 'do whatever you think is needed to solve the problem'? With the second option, there is some comeback if the problem re-appears...
Just a few ideas....