Thanks for the responses - I'm just back from the long UK holiday weekend.
It had never occurred to me that having all master pages defined in a single
template (I inherited it some years ago and have added to it occasionally,
but have never had to set a template up from scratch) could be
disadvantageous so it was interesting to look at it in that light. I see
that if I separated the various custom master pages into appropriate
templates, defining the standard Left and Right pages in each template
accordingly, I would get Apply Master Pages to work. Rick also pointed out
about the benefit of this approach regarding deletion of blank master pages
on save and correct application of master pages for new content.
And yet, my instinct is still to keep the master pages together - resistance
to change, I suppose, and the knowledge that there is just a single template
to maintain. Coincidentally, this Apply Master Pages issue arose in the
first place because I used to have a separate template for landscape
chapters, but now I need to cater for the possibility of mixed portrait and
landscape pages in the same chapter, so I thought it would be sensible to
bring the landscape pages into the standard template and save having to
think about separate templates, and then I thought that whilst I was at it
I'd take a look at Apply Master Pages. Turns out perhaps I should have been
heading the other way all along!
I'll mull over all this for the future rather than leap in straight away
with a few new templates as it wasn't a big deal to do the master pages
manually. But I now know more about custom master pages which is really
useful, so thank you.