Thanks everyone, for your advice. I did eventually get it working last night - Peter, I ended up using a method really similar to the one you described.
Let's say that now, for example(!), I had removed the ground or road surface under curb (as it was created by a CAD draftsmen who wanted to play with 3D for the first time!) and I want to recreate that now? Would I use a similar method, but head outwards instead of inwards, so to speak? This area outside the curb is not as important, as it will mostly be outside of the area that the architect wants to see, but I obviously need it to conform to the shape of the curb.
Again I apologize if these questions seem very 'noob', but I've always just fudged this to make it work in the past, and I'm on a much tighter deadline this time. Frankly I'm sick of stressing over it every time this kind of thing comes up! I really appreciate the help you guys offer.