Just make sure that you order it correctly.
It will be displayed from the top layers to the bottom.
So either make sure that it is below what you want to be visible OR separate it out as a new grease pencil object and then position it behind in 3d space. That way layers don't matter.
Hope that makes sense.
The separate grease pencil object moved back on the Y axis worked but the method as described didn't seem to allow for correct layer positioning, see the before and after screenshots, also changing the world background colour is a good workaround, anyway thanks mate.