Got both sides fully welded up! not pretty but they will hold
So.. just in case anyone else is doing the same and they're curious to how i did it, read on below. But i will say this, my way is definitely not the best way and it's not how they was originally.
The back plates for the hinge with the fixed nuts that sit on the inside of the inner wheel arch, they normal rot and fall apart! so before you do anything, inspect the back plates and see if they're any good. If your back plates was rotted into bits, then that means the inner wing that is sandwiched between the 'hinge back plate' and the hing it's self will be rotted, so then the best thing i could do was get a socket onto the bolts and turn them until all the nasty rotten bits broke, then you may have a chance of pulling the hinges, with what remains just straight out of there!
Now that the hinge is out of the way, make a cardboard template of the hole and use some thickish steel as you will want that to be the next back plate, cut it to the shape of your template and then reach under the wing to get the back plate in place of the hole, see if you can get someone to pop a couple of magnets across to hold the plate in place (done it on my own which was a bit tricky but doable). Once they're in place go a head and seem weld them plates in place. So looking in from the bonnet you will now see your welded in 'back' plate that sits about 2-3mm sunken to the level of the inner wing "sandwiched" steel where the holes would've been for the bolts. Now draw another template plate, which will be smaller as that will sit within the current seem welds but at level with the inner wing "sandwich" steel, and weld that in place, so you can have your hinges sit flat.
Sorry about my poor description, English in school wasn't my greatest subject
i will add more once i've gotten further myself