Help swapping Mk1 Focus battery

[F-Stop Junkie]

After the battery died in my 1.6 Focus, I've got a new one to swap it with, but the bolt holding the battery strap in place is just spinning and spinning. Had a peek underneath and there appears to be a mateal threaded piece in the plastic battery tray which is just spinning and rusted slightly.

What's the best way to get that bolt out so I can put my new battery in?