Just to add that after a promising initial phone call to W.Livingstones SEAT in Uddingston, the warranty guy there didn't even go as far as taking any photos or making any claim to SEAT. He took one look and told me, no chance they will honour a warranty claim.
He did very kindly give me a quote for a rub down and respray totalling £308
Considering there is a very clear line of bubbling directly following the path of the foam spacer, I'm highly miffed. I don't know if there's any point in chasing this, or if this is just part of the game they play with every rust claim.
I am just going to keep my eyes open for a similar coloured MK4 breaking somewhere that I can yoink the wing from. Or possibly just buy a new one and respray.