Ask your mechanic to DISCONNECT THE BATTERY and slide both seats forward and disconnect the two cable connectors underneath the drivers seat and the single cable under the passenger seat. (Easier if seats are removed) obviously check for broken wires!
The contacts on these connectors need to be thoroughly cleaned and then sprayed with electrical contact lubricant and the male connector pins should be expanded slightly to give a closer contact.
This is the most common problem which sets the airbag light glowing and is caused by a momentary interruption in the electrical circuit when the seat is moved with the ignition on.
While he is at it AND WITH THE BATTERY STILL DISCONNECTED FOR AT LEAST HALF AN HOUR get him to remove the plastic covers clipped over the inboard seatbelt tensioners and examine the wiring connection integrity to the EXPLOSIVE squib belt tensioners as they can get damaged/corroded. Also use your key to operate the passenger airbag immobiliser several times to clear any corrosion on the contacts (on the passenger side tunnel side trim panel)
I had all
these faults on my 98 GTV so this is from practical experience and it took ages to track them all down! I have not heard/read of problems with the airbag units themselves
Unfortunately this is not the end of the job as the light does not 'self reset' and you need to find some kind person/independent locally who can plug into the ECU diagnostic socket and reset the lamp. I expect that you have the early connection system which has three leads. You can check to see if, like me, you have the later single diagnostic plug by removing the panel covering the fusebox beneath the drivers dash (4 quarter turn fixings) and looking for a D connector with a grey or black cover at the right hand side just below the dash moulding.
I got my light reset by Autolusso near Luton but that probably won't be any good for you. Cloverleaf Independent in Gloucester may be able to help (01452 410970) if you fancy a blast along the M5.
Best of luck