Thanks so much for the top tips. The vacuum bag process looks to give a consistent shape to the formed leather. I have seen others hand sculpt and hammer the wet leather but think i'll paper mache a form of my B17, then cast concrete of something stronger to share the leather around.
I've managed to get my hands on an old stretched saddle so also am going to break the existing rivets and flatten the existing leather so i have a stencil to cut new leather from. All looking good i think i should have a finished project by the end of the week.