hi ya Paul
its not uncommon for fibreglass mouldings to move about a bit - especially if they have been pulled from the moulds too soon.
As it happens, long thin mouldings, such as you have in the Type 42 hull, the sides tend to pull in on themselves when they are still 'green' i,e,. when the moulding is pretty soft/not cured properly
- short fat mouldings do the opposite - the sides go outwards - if you have a large flattened area - you sometimes find that you get a hump in the middle when the actual moulding cures off. In your case, if it were me - I would put bracing inside the hull - say 1/4 inch square timber - along the inside of the complete hull & when this bracing has bonded in properly and dried - put tie bars between - made from the same material. Using large - either sash clamps or g clamps to pull the hull into the correct dimensions or push the hull out whichever the case may be.
aye
john