I just went through my garage looking for my 600mm shifting spanner so I could lend it to you but I think it's in the shed out the back ... now I read it's the hitch.
Heat the outside of the hitch and don't heat the receiver. I'd resist the urge to try hauling it out with a chain/strap because if it suddenly lets go, you have both a freed towball and a heavy missile in flight. If it's rusted in place, tapping it INWARDS will also help loosen the rust and once it starts moving, it should come out without trouble.
Failing that, I'd use Dion's advice. A press will slowly and SURELY remove it.
You may be able to use a bottle jack pushing inside the receiver, depends if you can get it in there and if there's anything to push against.