Stephen C. - This is actually a known bug in the latest version of Enscape.
You have two options:
Roll back to the previous version of Enscape.
Stay with the current version and observe the following procedure:
Set your resolution you want to render out at.
Do not open the Visual Settings window before a Render, but if you do open it, don't close the window. You can make adjustments int he Visual Settings window which will take affect. If you close the Visual Settings window, that is when the bug becomes active, and you have to restart ArchiCAD.
Developers are fixing THIS
Sounds good Pete. Thanks for your help.