Hmm... forget PTFE tape
My solution, in the end, was to use Lates' idea but with a thick washer glued onto the nut to act as a spacer so that the thread in the nut would then grip part of the post which was relatively undamaged. The DYH100840 specific items were useless as well. Much better to use a proper nut with a finer thread.
It worked perfectly
A little dab of UHU glue and a 10 minute wait for it to go off. Not TOO enthusiastic tightening them up of course
BTW, I wouldn't advocate trying to put a washer on the post and hold it in place while you try to start the nut... not unless you want even more debris inside the boot-lid