Sorry, I probably didn't explain myself well enough. I could see the rulers, but I was referring to creating my own rule or guide on the page.
I've now discovered that I need to go to View > Guides Manager and then set a guide on the page, which I can then reposition my moving it on the ruler (if that makes sense).
I was confused with InDesign, where you can 'pull' (drag) a new guide on to the page from the horizontal or vertical ruler. I thought it worked in the same way here, but I realise now that the functionality is different in Affinity Publisher.
Thanks for your reply.