oh ginge, ginge ginge.

have u done what i did with ur cossie one? used a bit of rope to keep the pully still?

best way to do it is to shock it.
big hammer, solid bar and give it a few wacks.
any movement is good movement, so even if u can do it up a bit it will break the seal it has and then it should undo.

the wd40 will help if u can get it into the thread. but sounds like its pretty tight so it might not work to great tbh.

stick it in gear as well, it will give it some resistance to help u undo it.