Trust nothing. I foolishly used the plank lines on the vac-formed boat hull as a guide when fitting the deck. As you can see from the photo above, it's wonky. Quite a bit wonky.
Removing the deck (Hint: Soak the joint in Plastic Weld and peel apart gently) and then using a big pair of dividers (thanks Grandad) to measure the distance from top of the hull to top of plank line and the problem becomes obvious.
I don't know how, or why, but the plank lines are wrong in the rear corner. They climb back up to the same level as the other side half way along the hull which is even weirder. Someone at the club had commented the hulls weren't symmetrical but I'd assume this was a just the typical vac-form hull errors (see past abandoned projects) and only occurred between the two sides, not along the length!
Putting in some new plastic support along the true line solves the problem and gives me a nice flat deck, but it's a problem I hadn't expected.