Just when I think I have made progress, the little ship bites. This is what happens when you take your eye off the ball for a split second.
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Take note of the position of the gunport second last from the stern. Now compare it to WB #1
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On WB#1 there is no problem, because the gunport lid is closed. On WB#2 however, the open gunport lid will foul the deadeyes.

And what is really frustrating is that there is no excuse for this blunder. I had WB#1 two meters away from where I worked, so checking that was the easiest thing in the world.
The only solution will be to close up the offending gunport and open the one most rearward (which will be strictly incorrect as I already have the two cannons mounted in the stern!)