Cheers guys, after posting I had a google (should probably have done that first ) and found the answer myself ie. no you don't need to disconnect them!
Not really knowing much about what set airbags off my thinking had been there might be sensors in the bumper, or that connected to the bumper, so if you were replacing the bumper you would have to remove the sensors etc. Having checked up on it though I see thats not the case though! :thumb2
I have wondered whether it would be wise to disconnect the airbag for when you drop into gulleys etc. I suppose it would have to be a huge impact to fire it.
It needs one hell of a crash to fire it I've dropped mine into the ground at considerable speed and attitude :augie bent the chassis and half the truck but the bag didn't orff :doh