just an update to say my son's UK passport arrived on Christmas Day, just as we were opening presesnts so the timing was perfect. Thanks for everyone's advice.
I sent then all documents and translations (including the Juri Shomeisho) minus the mother's book with an explanation as to why we couldn't send it. I was honest and told them my wife was dead set against sending it to them! Must have been the season of good will or more likely the other documentary evidence was sufficient.
So it all worked out in the end, took 3 months in total and a fair bit of fretting but all good.