Hi John, good to hear you getting out on NC a bit.
I think the ropegalore prices are pretty good, cheaper than whitworths & imho better quality(i dont think WW dble braid polyester is Donaghys), but the problem with ropegalore is the minimum quantity(dble braid) is 25M.
As far as im aware the outer braid on dble braid is just to protect, the inner braid that does all the work, so even if outer is furry, its still structurally sound, but doesnt look as nice(or cleat as well).
Im guessing

6mm(1120kg breaking strain) would be strong enough for a mainsheet but obviously that wouldn't be comfortable on hands, 12 or maybe 14 sounds about right, I think Blue Moon's is 14mm.
I was told once that the jib sheets on NC would be easier to use if they were the next size up.
cheers
Shaun