In reply to spenser:
Had a quick glance again at the bmc guide but its mainly for installing bolts not removing them.
The original Lakes bolt fund team had a diy extractor which rarely failed in its mission, now sadly misplaced. The team also had lots of other options for removal, some mentioned above.
If hammering, often better to cold chisel a nick in the bolt both sides first, helps the crack propagation process. If you cannot swing a hammer accurately, a flat ended bar will improve aim and damage the rock less. As others have said , it only takes a few blows up and down on a bolt before it snaps.
Buying a core drill, follow the instructions with a quality one. I bought a £30 silverline one, did not manage a single trial hole in soft slate .
If levering out , often the hardest issue is getting a fulcrum point close enough to the bolt to get the required mechanical advantage ratio. Also devising a method that works hanging on a rope rather than just at ground level.
There is also some US u tubes of methods to get out through bolts using a power drill and drawer.
Will DM some sketches of bolt drawers we have used.