In a report from Wood Newton it does say that movement in the wood has been an issue due to the sun we get, but latterly denies this has anything to do with people’s windows cracking on site (as quoted above).

Then again, we’ve been told that the water coming through wont affect the timber structure either. So the builders would have it both ways.

But as far as the curvature on the window shown in your pictures, I would think this is actually just warped trespass, we had a similar thing by our back door, the fix in our case was to just to add a couple of screws to hold it in place. But since there aren’t screws already on the window section this wouldn’t really work as it would look odd, odder then a curve where there shouldn’t be one? well thats a personal choice. But I wouldn’t personally think that the warp is having any effect on your issues.