However, you still didn't answer the question

........... regardless of whether I should or shouldn't do something a particular way, which is a kind of semantic/philosophic thing based on the latest fandangled ideas out of palm valley or wherever all the geeks hang out....
can I do it? At this point in my learning curve I'm not bothered about 'purity' and 'elegance', I maybe want something quick and dirty! I can refine the ways of doing it as I learn more.
Yes, you can do it. and here are the step by step intructions that should work (though I haven't tested).
1) go to 'Extensions >> User Defined Tags'.... Create a new user defined tag and call it include_php
in the text area that is supplied add:
(user defined tags are just php snippets that are stored in the database)
Click Submit.
2) Then in your page template, or wherever you want to include a php file you insert
I've spent the last two hours trying to unpick the relationship between the various templates and stylesheets in the default CMSMS site. And it's completely scrambled my brain! Pick of the bunch is a stylesheet called Navigation:simple_vertical, which can only be accessed through template associations and doesn't show up in the main stylesheet list; said stylesheet also seems to have had all its font styling separated out to somewhere else!! - where I haven't discovered yet. Simple!!...it's a nightmare!
In the various page templates, under "Layout >> Templates". You will see a css icon on each row. This will bring up a page that will show you the various stylesheets that are attached to that template. When you edit the template you will see a tag {stylesheet}, this inserts the code to attach the stylesheets to the resulting html when a page that uses that 'page template' is displayed.
CMS allows you to separate your stylesheets logically for re-use. You can put all of the css for forms in one stylesheet, for general styles in another, for layout in a third, etc, etc. and then you can mix and match which ones you use on your various 'page templates'... so that you don't have numerous large stylesheets with almost identical content.
I find the signature at the bottom of your posts most revealing - and rather at odds with most people's notion of simple, I think! To your credit you do qualify your idea of simple, but it's not the simple sold on the CMSMS home page!
Yes, at various developer meetings we've discussed the general way we advertise the product and who we market it towards. We've decided that we are marketing towards web professionals who want a tool that allows them to simply create a small to medium sized website with limited dynamic functionality, but extreme flexibility, and to do it quickly and easily after they learn a few things. My signature line reflects that.
I hope this information helps.