I de- stapled back to the 4th form, and cut thru the strongback and removed the bow stem. The mdf had warped, and using a trusty piece of string I reckoned the bow was out just over an inch. [this is over a kayak 17'6''..long..some might not notice, but to a trained eye......]
I cut through the bow and planed an inner bevel which I roughed up with a rasp, and reglued it with plenty of glue, after realigning the lot using the laser and a set of guides on the remaining forms.
I bunged the cut off stem back in after hacking it up to fit and banged a few staples in to hold it in place.
I will have to glue on a cheater strip as it is a bit lower on one side, but at least it's true now!
I haven't a clue what caused the twist in the mdf, maybe the transition from a different room.