Thanks for the very helpful advice and information.
It sounds like there were quite a number of things going on in the image.
The gradient being lighter on the city side matches the potential light pollution.
The aperture was wide open at f/3.5, so that matches the zoom lens vignetting.
There were some street and neighbour lights nearby hitting the lens, so that could have contributed.
I will try some of your suggestions and see what difference it makes.
