Thanks Ian, it will help to keep things square.
No-one has commented on the reversal of the frame and keel fitting slots. The keel is cut from the bottom upwards, half way, and the frames are cut from their centres downwards.
Because the slots are lengthy, I cannot reduce the inner heights of the frames because I'll run out of timber in the joint. Hence I have more height than I would have liked, giving a 'centre board effect' . You can see the partial loss of timber close to the red clamp in the last picture.
It looks like a ridge to separate the rowers !! so maybe I'll get away with it