Thanks guys, I really appreciate the help and experienced opinions.
I have spent a lot of my life working on boats so I'm pretty good with fibreglass. I'll take everybody's advice in here;
I'll cut a little more of the 2nd layer away as it's a bit crunchy rust and also it's sitting high now. Then I'll wire brush it all a bit to get off anything loose, then I'll soak the bejesus out of it all in rust converter. I'll make sure to get a little hose piece and feed it into all the holes and gaps between the roof layers. Then I'll fibreglass up the holes.
Then I'll get a pallet of filler and mix it with 3 buckets of hardener and hope that is enough to build up the roof corner.
Does that sound sane? Is one type of filler better than others for this?