Yesterday I decided to replace my rusting fuel filler ring bolts with new anodised black ones. Five moved and were removed very simply. The sixth moved OK to begin with then.........quelle surprise....I got that slipping (and sinking!) feeling.
Luckily, as I'd already managed to remove the other five, I could gently prise the ring away and create some pressure and leverage to help stop the nut turning so easily. After 30 minutes and copious amounts of WD40, the bolt started to free itself from the nut. What surprised me most though is that as the bolt finally came free from the nut, the nut didn't drop inside the clam. I honestly expected it to. But it stayed in place and the new bolt tightened up on it a treat (with liberal dashings of copper slip to ease removal in the future).
Anyway, the end result:

Now I need to consider whther or not to anodise the ring and the cap. But to be honest i quite like the black bolts against the silver of the surround.