Thanks for your encouragement, direction and help
Looking at the date of my post I was probably panicking - only a week to go to a public launch!
Sven's answer and the link to the technical forum solved my problem and increased my understanding.
The launch went well and Migration Yorkshire who paid for the development are now promoting it.
You can see the 1.0 version at that link and we'll be distributing it through the LearningPool route and a CC attrib NC license to anyone else.
As always I'd wait a bit for a 1.1 or above version - I've got some videos to add and tidying up to do.