Hi Tony - I've been doing that (the shimming) i'm making some progress, but I think the real crux of the problem is that i'm simply not taking enough wood off to remove the really high parts of the twist this is because the timber I bought was PAR and almost to my finished dimensions so i'm reluctant to take too much off.
I think i'm just going to have to live with the most difficult pieces, i'll see how bad they are when I come to the dry-fit.
Thanks to everyone for the hints and tips, as usual it's all helped!