Because I don't have a car lift and due to the amount of work required on the chassis, I decided that the best option was to tip the van, sort the underside out, then roll it back on its wheels. Using bales and a car winch I got my Dad to help and we simply took out the front chassis and rolled Rusty over!
Looking at the chassis properly it doesn't look as bad as first thought. I knew the back end was rusted and that either side at the from had taken some damage but other than that it seems to be a manageable project to sort. I was also relieved to see that the underside of the floor was in good nick and that it wont need replacing.