Hi Joseph,
Have sorted it out myself as you can see from attachment.
Recognised I had a problem, but because I'm unfamiliar with Moodle, didn't know what sort of problem it was.
Finally realised that, while I thought I'd created linked pages in a lesson, I'd actually made a series of single-page lessons none of which could link to each other. Then used the 'add a page' in edit panel to embed the pages within the lesson.