Hi a.l.e.
Thank you for taking the time to respond to my query. First apologies if I ask some dumb questions in the below.
The sample latex does not render in Scribus (or did you mean something different by 'markup'?). Other LaTeX (text, formulas, various formatting) does render fine in Scribus for me. Playing around in my native instance of LaTeX (on TeXWorks) based on your answer, it does not produce the file with pdfTeX but does with pdfLaTeX. I can also see from "Edit Source > Options" that I am using pdfTeX within Scribus, so I think you are correct in your diagnosis of the problem. (I used the "Rescan" button).
However (and here comes the dumb bit) I am not clear how to change this. Based on the above I think I need to get it to use pdfLaTeX instead of pdfTeX and I suspect that this is done through "file > preferences > external tools" as you suggest, but I don't understand how to actually do it. Could you be a bit more specific about what I need to do?