This is P*SSING ME OFF!!!! Oh it's going to get it now...
I managed to hammer a 12mm socket onto the bolt and turned it a couple of times before the socket slid/fell off... When I hammered it back on I knocked the nut off the chassis on the other side so that is turning with the bolt now... GRRRRR!
I'm going out now to snap off the head hopefully- i'm getting the cold chisel and lump hammer out again... Which always ends up in something getting destroyed.
Report back in a couple of hours.