I removed the seat in preparation for tomorrows repairs and gave the coaming a little tug. It seems the coaming has come away from the cockpit lip and is not a crack after all. The edge of the coaming and underneath of the deck had been gel coated white. Where the coaming has parted with the lip it looked like a split. There is of course the two hairline cracks to sort in the lip itself. I think the plan now is to remove the coaming, grind of the old gel coat and gunk, fibreglass the rim then glue the coaming back on.