Thank you for the advice.
I already have a logical printer set-up and that doesn't seem to have resolved the problem. However, upon experimenting, I have just discovered that if I go into the preview pane, click on set-up and verify the print setting, then press print from the preview pane, it will double-side. If I omit this extra step it single-sides, regardless of the settings defined in the print options, and regardless of whether I print from this pane or simply choose print.