I duly headed to Halfords to buy a new tub of P38 body-filler, but with big gaps and little for it to cling on to I also picked up a tub of P40 fibreglass filler, which is a bit tougher and bridges holes, as well as a small sheet of aluminium mesh to help form the shape of the ring and bind everything together. First though, to combat the rust, I threw on a thick coat of Finnegan's No.1 Rust Beater and stippled as much as I could into the fissures.
The aluminium mesh fillets were then cut out and pushed into place behind the headlight surround using the tacky paint to hold them temporarily.
I then began to mix the Isopon P40 fibreglass filler. I've not used it before and it turns out to be strong stuff which sticks to itself very well and, once it starts to harden a little, becomes really easy to form. I put a quick skim over the back of the mesh to fix it into place and then added a few extra mesh fillets to reinforce the inner edge around the top of the ring, which will be holding the weight of the top of the headlight.
I used two more slugs of filler of about the same size to nicely bind everything in at the back and fill the gaps at the front, though I may have been a bit over zealous with the thickness of the repair and it'll need a good rub down, not bad for a first try. The mesh really helped to make the shape - the filler clings to it superbly.
Soon as I find time I'll be rubbing down the rest of the wing's blemishes and breaking out the P38.