That is very annoying. It's pre-OBDII era, like my car so it has to go on the Rolls Royce Mastercheck system. Obviously, all the main dealers have this facility, but pretty much all the well known specialists also have it, as far as I am aware.
Your choices are:
1. Mess around with the relays again to see if that puts it right; maybe try disconnecting the battery for a few minutes; or, just use the car in the hope it will go away - which it may well do.
2. Go to somewhere with the proper diagnostic system and pay them to diagnose it.
3. There is a way of re-wiring the diagnostics socket to OBDII which has been described on here, so you could try than and then try and diagnose it yourself.
Personally, I would go for number 2, I just think it's worth it for all the messing around, and it will identify any other faults too at the same time which you may not know about.
When is the MOT due? You probably know, air bag light is a fail and you can't just disconnect the bulb because they look for them to all come on with ignition and then extinguish, so if it doesn't come on at all they know it's not working, so that's a fail too! So it is a real pain of a light to come on.
Good suggestion, however, I think the idea behind the original suggestion was to de-power (0V) the system to see if the air bag resets.If you have to remove the battery, simply add another in parallel to the old one with jumpers. The car will not notice and your memories will be saved.