Originally Posted by CosRush

If your careful, using a hacksaw cut a screw driver slot in the top, and use a decent large head screwdriver with a socket body (the ones where you can use a spanner on it).

The apply loads of downward pressure by leaning on the screwdriver head and with the spanner on the screwdriver body turn the screwdriver.
Would be an idea to spary some wd40 or similar on it first ensuring it seeps down any minute gap around the bolt.

Plus, to be safe, get a scrap piece of sheet aluminium (small piece) and drill a hole the same size as the top of the bolt. then place this over the bolt head and should you slip off with the screwdriver you'll hit the sheet metal and not the block.

hth