Hi Graham welcome to pixls!
It has been a long time since I tried using the print module. I remember there being an alignment tool in layout. I assume you double checked that is centered? I assume you double checked your margins?I also seem to recall you can add more image to the canvas, and it reserves space until you delete that area. I don’t know if any of that is helpful.
Very few people print from darktable (for a host of issues not related to dt specifically) and without a recognized printer plugged in you cannot access the print module. It may take some time to get a response.