@Ray Gibbs: The timber is closer to 15ft (around 5m). I've already taken smaller twists out of two girts with the aid of a scaff tube, but I'm twitchy about this much twist.

@The others: Thanks for all the tips. We have an alternative solution (of sorts) which we think might work. We'll need to plane the ends to get a reference (probably). Then (as we're working to square rule) we'll position the ideal timber so that the reference face is essentially outside the real timber, offsetting it downwards. This will move the twist out of the range of the floor joists which means that the floorboards will never intersect with the girt. If I can work out how to attach files, I have a SketchUp file which helps explain this. And here it is!

This of course assumes that I can cut the joist pockets in the correct plane (the ones in the rest of the frame were cut with a router as the faces of the timbers were as near as dammit in the correct planes). Snapped lines to mark the bottom of the pockets should be easy enough to follow. Oh well... I can't claim that I want to do this as a career in the future on the one hand, and get upset about a slightly wonky bit of timber on the other! :-) Enough contemplation. Tomorrow I'll get on with it!