Thought about the 5kg bags of chopped strands overnight and realised that 5kg of chopped strand mat costs less than £5!
Anyway, out with the angle grinder this afternoon and removed the 5kg of body filler that I had used to try and build out the off-side/front of the main tub. Then I ran some strings along the side of the tub at various heights and confirmed that the lines of the near-side of the tub are pretty straight - lets say 'Sammio' straight. Whereas there is a definite gap to be filled-in (as shown below) on the offside.
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In fact this photo under-eggs the gap because the piece of wood is only about 18" long. Running the string along the side of the cockpit shows the gap to be more like 10mm + as per below.
So having ground-off the filler I spent the rest of the afternoon put a number of layers of mat to build-up the area between my 2 x correx spacers.