If you don't want to pay someone, probably your printer, to format your book, you have to do it yourself. Luckily I bought an ebook on how to be an independent publisher, which, among lots of good, detailed advice, told me what to do. I still have to check the final details with my printer, but I've done as much as I can to keep costs down.
The ebook I mentioned is called Smart Self-Publishing: Becoming an Indie Author by Zoe Winters. She is American, but very little is different here in Britain. I plan to post a detailed review later, but you can link to the Amazon page for it from its title above.
I didn't know what size the pages should be - it is now formatted to standard paperback size. I hadn't thought about what they call 'front matter' - the pages containing the title, author, copyright notice etc., and the Contents page. Until I had done that I had no idea how many pages it would be and thus how thick the book will be. At the moment, it looks to be about forty pages, but I have allowed half a page for each illustration and that is subject to negotiation with the artist, also how many illustrations.
Thanks to Zoe Winters, I can now see the light at the end of the tunnel.