Thanks very much for your prompt reply, which reassured me that I was on the right path. From having read various Help topics, I suspected that I'd have to use an Excel data source for the numbers. Fortunately, my knowledge of Excel was good enough to know about the drag and drop facility to create automatic sequential numbering, so the data source creation was easy. In the end, it was the mail merge (no surprises?) which proved a tad tricky, but I got there in the end. I've used MM many times and quite happily in Word documents, but for Publisher label format, it was of course a bit different. The important bit that I had to discover for myself was the significance of, after getting to the Print stage, going into Print options, to Publication & Paper Settings, and selecting Multiple Pages per Sheet (& in my case, also "Single-sided" cos my default of duplex printing had come up). But TA-DA!  All is fine now. Thanks very much again.
I’m unable to access Numbering & Section Options from the Master page (it’s greyed out). I had already manually ‘told’ all the files in my Book where to start, using the Numbering & Section Options from the Layout toolbar (I didn’t know it could be done through the Pages menu). They still show up in the Book listing as starting from page 1, though. I had to juggle between the Document Setup on the File toolbar and the …Options from Layout to get the page numbers to come right. As I have a lot of illustrations throughout the Book, I’d like to have the correct page numbers come up so I can easily find and acknowledge them.
My quick process involves setting up a row of plaques with a01, a02, a03, thru a09. Then copy the plaques to the clipboard. Run the REPLACE TEXT feature to replace "a" with "1". Move row down and paste clipboard back into page. Run REPLACE TEXT to replace "a" with "2". Repeat until I get all the numbers. It's faster than manually typing the numbers in but I know there's go to be someway of automating the process.
And what all did I forget to do? I forgot to go to 'Styles', 'Paragraph Styles', and I went, right click 'Redefine' because I've reverted to what it was before. So that's page numbering, we used A, B, C, D at the beginning. And we're going to use 1, 2, 3, 4, 5 for the rest of it. The only trouble happens, is when you go 'File', 'Print' now, and you're like, "Hmm." "I want to print pages." and you're not too sure. So what I have to do is go to A, through to 30. So it will include A, B, C, D, E, and all the way through the numbers. What if I just need A, B, or if I just print 1-30? It's going to skip out these first pages. That's just something to be aware of when you're printing, or making PDFs, that the page numbering is a little bit stranger now that we've gone and messed around with the page numbering.
To include numbering prefixes from higher levels, enter text or click at the start of the Number box and choose Insert Number Placeholder and then select a Level option (for example, Level 1), or enter ^ and then the list level (for example, enter ^1). In a list with first levels numbered 1, 2, 3, and so on, and second levels numbered a, b, c, and so on, including the first-level prefix in the second level renders second-level numbers as 1a, 1b, 1c; 2a, 2b, 2c; 3a, 3b, 3c.
You can insert page numbers on the current page, all pages, all odd pages, or all even pages. When you insert page numbers on multiple pages, a new master layer is automatically created, and the page number is placed on it. The master layer can be an all-page master layer, an odd-page master layer, or an even-page master layer. For more information about master layers, see Creating layers.
Changing the numbering display affects how pages are indicated in the InDesign document, as in the Pages panel and in the page box at the bottom of a document window. The numbering display also affects how you specify page ranges when printing and exporting the document. However, the numbering display does not change the appearance of page numbers on document pages. numbering in indesign