So, after having a chat to a fellow rally driver/panel beater I’ve got some clue as to how to go about this (I did say some, I apologize in advanced to all of the panel beaters out there who will probably be shaking their head if they ever read any of this!).
First task was to cut off all of the bits that are not going to be savable.

More has to come out still, but I need to pull the bottom of the A pillar back into place first, using what is left as a rough guide of where to stop. So the next job is to make a frame to mount the car to, and a pulling arm to pull things back out – I feel a YouTube training session coming on.