Hi there Arne and David,
Thank you both very much for your comments. My initial thought was to go with the heavier V92, and although I really see where Annie is coming from - being that there are much lower stresses on the cloth and seams in a junk sail - nonetheless I'm thinking it does make sense to go for higher volume in the thread itself. I'm glad of David's comment about coloured thread, as it reinforced my view that a brown or dark red thread might look good with a cream sail so we ordered some "burgundy" V92 from PT Winchester.
Fortunately the sewing machine came with a 110 needle.
Thanks Arne for the two links to the making of ED's sail. Sewing in horizontal panel seams seems to be a logical method, probably quicker to do and offering a bit more protection from chafe. Our "half lenses", as you put it, will be a bit wider than on ED's sail I think. Amanda has made a few scaled down mock-up templates, and has marked out two or three panels on some ancient calico that we had knocking about, so sail making has started! We are still waiting for the village hall to get back to us on how much they want for us to use the floor...
Gross extravagance here: I couldn't find non-stretch white or cream webbing apart from one (but the seller could not vouch for its non-stretch qualities) so I ordered Dyneema which is only 25mm wide. Now that I have it it does look a bit mean, but will it be OK? Of course stretch is not going to be a problem, but will the lighter webbing - compared with the weight of 40 or 50mm stuff, be sufficient stiffness in the luff/leech?
Apologies that my text seems to be disappearing off the edges of this post.
Thanks again,
Pol.