start with only VFR, too many changes at once will confuse us!
if no change; disable the next inline till you find who/what is involved first

Might have found the culprit: The VFR Real texture. I disabled the photo, but left the mesh, just for the northern England area. The southern border runs in a straight line which bisects Manchester airport. Look at the top down view with the Cessna on the active. Below the line, there's photoreal scenery, above it, just mesh and default textures/autogen. You can see the NE splash map being excluded in the semi circular zone centred on the plane.

 

After taking off and flying north over the default texture, the exclusion zone was not present, even when panning around in external view.

In the VFR Real blurb it does mention night lighting effects (custom night texture somewhere?), but they're nowhere even close to matching NE! 

way go Mark!!!

Yep - the VFR Real scenery has their night layer embedded within - can't disable that without disabling the whole photoreal scenery - a cautionary note for other VFR Real users.  As I mentioned in a previous post, on the bright side (pun intended), the NE lights are still there, but not the splash map.