Thanks JFisher. Yes, I think you are on the right track. I am using an NVidia GTX 1050 Ti graphics card with two monitors attached. I use the Surround feature of the driver software to combine the two monitors into one display. Disabling this feature and just using Windows settings to span my desktop across both displays allows Affinity Publisher to work ok but unfortunately this results in some other programs not working well and a Task Bar on both monitors, which I don't like. Interestingly, it worked fine with Surround and Publisher before I changed my two monitors. Under the old setup, the monitors were connected via a Display Port connection & DVI connection but the new monitors are connected via an HDMI connection and a Display Port connection. As far as I can see, this is the only difference. Otherwise it is the same graphics card and same software.
I like the Surround set up, so I think I am a bit stuck. Big shame as I have been a fan of Serif Page Plus from it's inception and was liking the new Publisher. I suppose I can switch Surround off to use Publisher but then I have to set it all up again afterwards, as the settings aren't retained