Adding Text to Layout

Although this tutorial is for iPhoto it should apply to Photos as well: iP11 - Creating a Custom Page, with the Theme's Background for an iPhoto Book

One way to apply the tutorial is to create the blank, colored page and add it to a Pages layout that's 8.5 x 11 or whatever size you're going to order. The you can add photos, resize them and place wherever you want and add whatever text you'd like.

Am I right in thinking you are basically building the page structure outside Photos and then just essentially copying it in ?

Yes. That's the only way unless you use a 3rd party boot creator on line like PrestoPhoto. However, I'm not sure it allows full modification of pages. You'll have to check to see if it does. It can create a book from a pdf file that Photos or iPhotos creates.

and AFAIK there is no facility to either paste a text image in or add a text box anywhere on the page ...

That is correct. You have to use a theme for the book, that has the page layouts you want to use for the book.

And all themes allow at most one textbook per page. You cannot add additional text fields.

Is there any add-ins that allow you to do this with Photos in book creation if Photo's itself cant do it? Or can you import other page option layouts somehow?

Photos 3,0. offers now project extensions for Photos. You could order your book from a different print service, if the service offers photo books. But so far, I cannot see Mimeo Photos for creating books.

Type appex:com.apple.photo-project into the search field of the AppStore. It will show you the third-party options for photo projects.