the air bag light comes on when starting the engine up then goes off?

All the lights on the dash (engine management, emmisions, ABS, airbag etc.) should come on with the ignition. This is to let you know the bulbs are working. Some will go out after a few seconds (once they perform a self-test) others will go out when the engine is started.
As long as the light goes off the airbag system is OK.
If the light stays on the system will be disabled.
If it does stay on the car would not fail an MOT, it is perfectly legal to drive the car. However, if you were to have a crash none of the airbag/SRS systems would deploy.

hth
November 2009