Driving down the motorway about a week ago and the speedo started fluctuating then started to work intermittently then the fuel gauge started doing the same and now the airbag light has come on does anybody know of a common fault or area to look at

I'd suggest checking any relevant earths first, if not it sounds like a dashpod fault.

Any other electrics (lights, rev counter, radio, ventilation system, ABS, traction control) playing up at all? If so, you may well have an intermittent low alternator output, due to the sprague clutch slipping.