Timmo, Ade, and others -
First of all thanks to rayclyde [Clyde Williams] who suggested reducing the Scenery Complexity setting. It does get rid of the intruding
default stuff, textured and untextured.
I have Rob's VLC Nelson, and could see nothing 'wrong' with it, certainly no black buildings. So
being a stubborn know-all sort of bugger I tried all sorts of other things first.

My original setting was Scenery Complexity = Very Dense. This caused problems at these airports:
Wellington NZWN - three black buildings and four textured intruding. Scenery Complexity = Normal gets rid of them.
Ohakea NZOH - two black buildings and three textured intruding. Scenery Complexity = Normal gets rid of them.
NB - Neither of the above is in the VLC Airfields package.
Gisborne NZGS - part of the terminal model appears as a black building. Reducing Scenery Complexity (even to Zero) has no effect.
Whenuapai NZWP - numerous intruding buildings, both textured and untextured. Scenery Complexity = Normal gets rid of them, but it also gets
rid of the 3D cars, which I can live with, but I would rather have them than just coloured blotches on the ground.
Whangarei NZWR - I hadn't looked at this until I checked it before trying Lawrie's new one. The terminal model was mostly black. I then
installed Lawrie's (because installing a new airport at Invercargill had solved a black building problem there) and ran into a shoal of red
herrings....long story short, I finished up deactivating, deleting and uninstalling VLC Airfields and Lawrie's NZWR. I then reinstalled both
and finished up with Whangarei looking as Lawrie meant it to, even with the Scenery Complexity still at Very Dense.
Sad to say, this had not changed NZGS or NZWP.
So I finally tried Clyde's suggestion, with the results described above.
I no longer see 3D cars at Rob's VLC Nelson, which is a shame. There is one at Gisborne, and heaps of them at Whangarei.
I moved the VLC Libraries up from 67 to 1 in the Scenery Library, without effect.
So it seems Scenery Complexity (on my machine at least) should not be set greater than Normal.
I am still a bit mystified as to why this setting, which should have a global effect, disappears the cars from some airfields but leaves them at others.
Many thanks to everyone who helped.
MikeW