Thanks for the tip, also to @aurel42de for the above. Much appreciated 
Going back to editing tonight, I followed the tip in this post to suppress the original scenery (e.g. extra windsock) while editing, but now have a new problem. My loaded WIP scenery is ‘below ground’, meaning that all of the placeholder lines & nodes showing taxiways and parking spots are clearly some metres below the surface when moving the camera around. If I attempt to move any node, it jumps up to the surface.
Anybody else had this situation arise and if so, how to rectify? (Except NOT selecting the option to load my scenery exclusively, which is what I’ll do for now…)
Edit/update: After dropping to the menu, reloading and doing the exact same steps, it’s now OK. 
The ground does drop a few metres when first loading the project, before I load the BGL in the editor. Guess it’s normal though.
Second edit: Opposite issue in fact. The runway outlines and some aprons edges and nodes (the ones which I did not edit) are now floating above the ground. Textures are rendered on the surface. Gives me less confidence it’ll reach approval… but it’s a learning exercise. (Sorry to derail the thread…)