I created a new template for a book but when I paste in the original text and images the new pages revert to the original A4 format
What is your definition of “template”? Is it a “standard” .ott file?
When you paste something across files, the pasted fragment comes with its original styling, which means it also imports page breaks with designated page styles.
If you’re trying to work with LO templates, read first the chapter about templates in the user’s guide.
Anyway, describe in more detail what you’re doing and what you expect.