So the two offending planks have now been removed (with a hot air gun as suggested by Chris). No chances are being taken second time around, with each mould removed one at a time and slots cut to receive two ribbands running the length of each side of the hull. One ribband to define a fair curve for the top of the sheer plank and one to pick up the lap between this and the lower plank.
Like so many new jobs one learns from experience, and if we were starting off again I would follow the method recently described very well by Chris Perkins in Water Craft magazine (no. 73 Jan/Feb 09). Chris reduces all the mould sizes by the thickness of the ribbands and fits these along the course of the ‘lands’ between each plank overlap. Whether or not this would be different in thicker ply I’m not sure but with 4mm it is well advised.