Quintin,
There are some typos in your master page mapping table. Look at the Console pod after you apply master pages and note the line:
One or more master pages do not exist. Aborting "Apply Master Pages..."!
That message refers to the entry near the bottom for E:BackCover which applies master page BackPage_Core. I assume you meant Backpage_Core. Changing the 'P' to lowercase and then applying master pages results in the No_Header_Footer_Right/No_Header_Footer_Left master pages applied to the entire document.
Your entry in the Range Indicator column in the last row is "Span" it should be "Span pages". You've misspelled "Single" in that column a couple of rows up.
I have tried "Span pages", "Single", and "Until changed" in the last row and get the same results with all three. I'm not sure why.
I don't have time to test now, but I believe "Span pages" is a variation of "Single" and refers to the case in which the first paragraph in the selected element spans multiple pages.
--Lynne
I feel like a complete muppet. Huge apology for wasting your time, Lynne. Turns out I did have a typo in one of my MasterPage names in the mapping table. Fixed it (and the typo in the Range Indicator column) and it worked. Talk about not seeing the forrest from the trees!