Did the guy you bought it off have a flat nose? seriously most on here (inc me) have no idea what's actually inside them, theory is OK but that's as far as my knowledge goes. I presume there is a cylinder of gas that over time leaks out, is it not possible to have these changed or recharged?
I had a quick look on line and I can get a new drivers air bag for £114 which to me seems about reasonable, but it states they are original MG Rover so does this mean they too are 15/20 years old. Passenger air bags and the seat belt pre-tensioners both seem to be unavailable new so it's SH (dodgy) or recon if available.