I just keep on learning new stuff!
Flash units have a GN number. You can use this to determine your aperture. If you divide the GN by the distance that the flash is from what you are photographing, it gives you your aperture.
GN numbers are in feet and also in meters, make sure you use the correct one!
Aperture controls the flash. The shutter speed controls the ambient light.
So, you set your shutter speed to how much light you want to show in the background, and then use the calculation to define the distance of subject from flash (GN/A). Too fast and the shutter will not fire in correct sync with your flash, and you get dark lines through your image. Set your aperture to suit. You can set an SS to be as fast or as slow as your flash can handle. For modern flash units this is around 1/200 or some up to 1/320.
Use manual mode - it gives you more control. Set the flash to manual, and your in-camera flash to manual, but turn the camera flash down as low as you can, unless you want front on light.
This way you can turn your flash right down, and use an off camera flash to create what ever lighting effect you want.