Turns out the nut wasn't properly galled. It had stopped at some thread damage but unscrewed easily. It appears that because of the thread damage the bolt hadn't been properly tightened hence it was slightly loose. The bigger risk though was from the nut which didn't seem secure. Despite having come prepared with shovels and rakes and implements of destruction it was quite trivial. I removed hangers and chain from both bolts and left the studs for now. That will give a bit of space for a bit of a chat about whether these anchors have a future.