Rich,
Do you mean airbag ignitor or side airbag sensor?
The ignitor is part of side airbag module in the seat, side sensor is in the floor (somewhere). I assume your airbag light is coming on the dash? - this shouldn't be an MOT fail.
BTW, unless it is a loose wire, these aren't servicable parts, so brace yourself for a new side bag + labour.. OUch