Thanks for the replies. The book is copyrighted so I've used the (completely new to me) plugin. There are a number of bits and pieces but the most 'impressive' is in the ch04b.xhtml part.
After the line identified as 51 in the Calibre Editor the first line of text is displayed correctly but then the next line is a random collection of dots as though the line has run off the bottom of the screen. The next line of text to appear is that marked as 77 in the Calibre Editor.
Looking forward to learning something, hopefully . . . .
Moderator Notice
You need to use the scramble book PI and not post Copyrighted material in the clear. Removed