So... it's taken me longer than planned to cut the frame and in time, my nice straight DF timbers have dried. I've already taken the twist out of two girts with the aid of a scaff pole strapped to one end once the other was housed. This worked for correcting a minor twist, but I've both girts were laid out pre-twist. I'm not sure that I can apply this method to the current one, and I'm scared that if I lay it out on the assumption that I can, then when it comes to fit it, I'll be screwed.
So the issue then is, how to deal with this timber. It's for a girt that has joist pockets and will be ultimately supporting a floor so I'd prefer to not have it two twisty once in place. Right now my best guess is to try and plane the top surface back to true and work from there. The problem is that the top should be a reference surface and I'm not certain that I can plane it to true enough to use it as a reference. I don't have any way of getting a photo online here (working on site), but I'll try and upload a photo of the beast tomorrow night. It's the only really headache that I've had so far, but I can already hear the creaking of the floor that ends up sat on it.
I guess the alternative is to move the problem and just scribe the floorboards to fit... hmm... :-)