: I found it for £37 for 10 sheets of 6mm x 1250mm x 800mm
: Also, it was the ease of construction - being able to wrap some sheets around
: the forms that interested me! Also, I'm stuck for room to actually mill
: all the strips I'd need if I used wood.

If the Depron is flexible enough to curve the forms in a full sheet, it will be too flexible for the usual form spacing. Two methods are, more forms which would have to be worked for shape, or temporary stringers set into the edges of the forms.

Cutting the Depron into strips and stripping it like for wood would need more and closer forms to support the strips.

I was looking at the site and notice that the surface or the sheets have the smoothness of a mold finish, the 3mm is the same weight as 6mm, the difference is just the mold face pressures. The smooth surface will have to be taken off somehow or the glass/resin won't have a good adhesion. Light sanding may do it but I've never had much success sanding polystyrene boards. Never handled Depron, though.

If I was going to use something like Depron, I'd get the 3mm and use two layers of foam glued together in strips, offset by half the width of the strip. The 3mm is twice the density as 6mm so it would be stiffer in two layers to make 6mm. This would make the core twice as heavy, (you might need more than two fingers and a thumb to lift the core)

I'm suprised to hear the difficulty in finding relatively inexpensive cedar in your part of the UK, but you don't have to use cedar. Clear white pine would do fine. You can work it thinner than cedar for the same strength so the weight difference isn't noticable in the final kayak weight.

Mike Savage
South West Cork