Very pleased to say that I have just received the following reply from the Nazeing works...............
""Hi Paul, Yes we can confirm it is a Nazeing product as we have one hanging in our museum. The colour is unusal but we did occassionally use it.
Kind regards,
Vicky Payne
Technical Sales Executive - Nazeing Glass Works Ltd""
So just goes to show that my pessimism was unfounded

My sincere thanks to Nazeing for their prompt reply and help, and especially to Vicky Payne for allowing me to quote her reply. I understand that the colour is 'probably' sapphire blue, and the period of manufacture was somewhere in the 1950's, which according to the book makes it the 'second period of manufature', probably. The first was pre 1940, I believe, although exactly when the 'third' and final period was I'm not sure.