Ok, the bolt is moving, you're 99% of the way there.
1) Undo the bolt as far as you feel comfortable with.
2) Cover the bolt in whatever lubricant you have to hand. WD40, plus gas, goose fat, doesn't matter
3) Do the bolt up again. This will draw the lubricant down into the threads
4) Repeat until the threads are fully lubed and the bolt turns easily.
5) Remove bolt.
I agree that a sheared bolt is a total pain, you could probably drill it out with a right angled drill head, but let's not go there.
Honestly, a ratchet spanner isn't great for high torque, I generally use a set of extra long combination spanners when I need a bit of brutality. IMO yarding on an M8 bolt with a scaff pole is asking for trouble!
edit - seen Grahmeo suggesting the same idea, so it must be the right answer!