Where are the templates for the Compile Report function?

When I am writing a file of R code and reach the point when I wish to record the code and its output, I use the handy button 'Compile Report'. This brings up a window headed "Compile Report from R Script" and below gives the option of various report formats, one of which is MSWord. I would like to amend the template for that option, but I cannot find where the template is located.
(I am well aware that with a bit more work I can get reports on user-defiend templates: I just want to avoid that bit more work! A single button is really useful).

Word output is handled by pandoc so I believe what you're looking for is in the guts of pandoc (and I'm not sure it's a single file — but there are others who know a lot more about pandoc's internals than me).

But couldn't you could still use that single button with your own template, as long as you include the template reference in a YAML header comment in your script? (you could make inserting the YAML easy by creating a snippet for it)

Many thanks for such a quick response.
Possibly what you suggest would work, but I am looking for a one button solution that does not require me to write any YAML in my script at all: I just want to amend some standing template.
The main thing that is bugging me is that the MSWord template assumes US Letter page size, and, being based to the east of the Atlantic Ocean, I want my pages to be A4. So I am looking to edit some MSWord template that must surely be in a file somewhere so that I do not have to think about the question again. As matters stand I have to 'duplicate' the MSWord document, delete its predecessor, change the page size, and save it again.