That Spitfire is one fine plane.

I like steam. I've built tons of warped wings, some worse than others. Fully-sheeting wings are tough--the warp's kind of locked in there. But water will work wonders. Steam is good for smaller pieces, for whole wings I'll use a bucket of hot water and a washrag. Really soak it in. Then weight it for at least a day. I've never had to cut the sheeting. Sometimes I overdo it and it warps the other way, but usually it's not too bad. If I can get the twist down to half an inch or less, I can usually get the rest out with the covering (same way you add washout to trainer wings).