Nick
As Griffo says, this is a really tough one. I don't know the Duratec engine well so my comments are based only on generalities.
To be 100% certain you need to investigate this further. I don't know if you have access to an endoscope, but that might be able to look at the threads more closely and ascertain if it does require a helicoil.
Can you remember if the number 2 plug came out with less effort than the others? If so it might have sitting "loose" in the thread , but I doubt this. I suspect it was reasonably well seated, but disturbed when removed.
In the end , to be sure, I think you are going to have to bite the bullet and take the head off.
Good luck
