The rigging on my Sopwith Camel seems to have softened up badly since I fitted it and took the photos last week, and as mentioned before warming them with a hairdrier seems to only work temporarily no matter what I try, so I've decided to bite the bullet and have stripped it all off, and started again with the finer nylon 'invisible mending' thread.
I tried making thread from stretched sprue, which worked fairly well but I couldn't quite get the thickness consistant enough, so decided to try the very fine nylon thread.
I've completed one wing of bracing so far, it is easier to fit than the thicker thread I used previously, and is looking good and taught, but I think I'll leave it a few days now to see if anything changes over time. The only down side is it is so fine, but hopefully after painting it a darker colour it'll show up more effectively.