Sounds to me like you might have printed the book to PDF using Print Booklet to make your imposition, or perhaps you just checked the "spreads" box in the export dialog. Leave the book as-is, do not use Print Booklet, export to PDF (Print) using the settings the printer recommends, and uncheck the spreads box. You will get a PDF consisting of consecutively ordered single pages, which is what your printer needs.
Thank you Peter.
I save the book to .pdf via the File>Export menu. There is no print booklet option there. Your post (and my desperation), encouraged me to experiment a bit more.
There are two dialogue boxes to set up. First the File >Document Setup, then File>Export which gives the .pdf setup box.
File>Document decides whether I get spreads or single pages by ticking Facing Pages box. The .pdf dialogue box offers pages specs for All, Range or Spreads.
I found the answer is to tick Facing Pages in File>Document Setup,
Then In File>Export tick All, leaving Spreads un-ticked.
My mistake was to have Facing Pages unticked in the File>Document setup box. That gave pages numbered as single pages.
Thank you for your help. You got me thinking and that led to a solution.