I have a 1998 XJ V8, every warning light up to yesterday came on and went out as they should, now I have the Air-bag warning light comes on when ignition is switched on, goes out after starting up, then comes back on again and stays on? (funnily enough, it seemed to come on after my wife moved the drivers seat forward?)

Could this be a plethora of possible causes or is there a relatively simple reason for it? Please be gentle, lol, new to this car and its complexities.

 

You're in the wrong forum. Post this in the X308 forum, the one for your car that was made up to 2002 when the car for this forum came out.

 

This is the forum in which you should re-post this question:
XJ XJ8 / XJR ( X308 ) - Jaguar Forums - Jaguar Enthusiasts Forum

Also, not a diagnosis, but I've found this happened in my older car, and it was a matter of reconnecting the harness.