Let me think it through and see what I can come up with. I realised as I typed that the PDF generator could not use the standard custom WP templates. It is a shame really, and is the fault of WP - the way WP has grown over time makes some things much more difficult and cumbersome.
However, I can create a *core* custom template script that just delivers the *content* of my custom page. That script can then be used by both the PDF template and the standard WP theme (I hate code duplication - I like to write things once ;-)
What I was wondering, is if there was some way to handle custom templates in a better way, so the PDF generator could see that a page uses a different template in the normal WP theme and so could see if there is a similarly-named template in the PDF theme and use that.
What I am using my custom templates for is to generate content - not a list of pages or a single page with a short-code in - but real unique content.
Now, just writing that, I wonder if my best approach will be to pull all my custom content-generating code out of the custom template, and put it into a short-code instead? That way my content would appear within the standard PDF default page rendering. I'll try that. Thanks for listening :-)
For interest, this is the page:
All the tabulated data here comes from a custom post type, and is divided into a series of tables according to a custom taxonomy. With ALL of that simply hidden behind a short code, it should work like a trooper with your PDF plugin.