I`ve just unseized a bumper adjuster, woohoo
Luckily one side came off easily so I could see what was going on..
Only the bottom half the bolt and bracket is threaded, the top of the bolt has a massive recess that seems to be designed to trap water and rot the first 3 threads...
So in the recalcitrant one I drilled 3 x 3mm holes about 1cm from the top of the bracket, giving access to this recess, then set about beating the bracket and the bolt with a medium hammer (hard, but not enough to distort anything..) this broke up a lot of the rust inside, some of which fell out the holes.. *eventually* it freed up and started to move
Right king canute of a job though..