In response to Graham Burton's question about deck/cabin composition, the best source of hard information I have come across - as a seeker of truth, given my task of rigging from scratch - is from Julian Merson's own Daisy III blog which includes a full photo library of a Baycruiser 20 in construction (not Daisy III). One photo of the roof is attached and this shows what a complex structure it is : some is airex/ ply sandwich, like the sample from Julian's cabin bulkhead, while other parts where standard high load fittings are attached (eg tabernacle, organisers, genoa tracks, clutches/cleats) are solid ply. The hull and its reinforcements, including the tanks and cockpit, appear to be entirely fibreglass sheathed ply. The coachroof sides and deck are less clear, to my ageing eyes at least.
As to the deck paint, since I have rather more to cover than Julian Merson's scratches, I think I'll stick to 2 pot polyurethane. Swallow now use Jotun Flexi Hardtop cream (RAL 9001), so I will give it a go...if I can establish a reasonable colour match in advance.