Thanks for the replies, guys.
Not sure that pics would help - the captive nut is inside the front chassis member?? I haven't tried a batt gun since I don't have one.
I would try and get at it by taking the bumper off but since the nut is still captive and it is the thread that has stripped I am unsure how that might help me?
Is there a general technique for removing bolts with stripped threads e.g driving a screwdriver under the head of the bolt and applying pressure to pull the bolt out whilst rotating with a spanner. That is the only thing I could think of.
Any other ideas welcome. Ta!