I have been remiss in posting, but I have installed the fly zipper and facing, using the tutorial from hot patterns. I went as smoothly as it did the first time I used this, until I realised that the tutorial was based on a woman's garment - men's openings go the other way round!
I've decided to leave it the wrong way and hope no one notices. Undoing it would be a real pain, and probably damage the fabric at the centre front. Other than being the wrong was round, it work well, and the zip is nicely concealed from view. The next step will be to put it together so that I can fit it before continuing.