Hey Malcolm! I had the same thing happen, but the previous owner caked mines, and it shattered like glass. I suggest removing the headliner, and using a slide and hammer, and then a ball peen hammer, i forget how its really spelled tho... tap tap tap, making sure its punched back out, or at least to the point of grinding it down. From there, try using the cute ball peen to really smoothen it out as much as possible. After attacking the scar from the inside, work it on the outside, grind, and use the proper filler. Metal filler, or fiberglass filler. Bondo is onlygood for baby dents and small scars up to roughly 1/4".
Slide tool
Ball Peen Hammer
It pays off to have bodyworking books laying around. best of luck man!