I have managed to answer my own question, fortunately! So I'll share for the benefit of anyone else who might come across the same issue.
The simple truth is, the terrain, whilst it looks perfectly flat, actually slopes by about 1 foot from one side of the construction site to the other. So when you put a building on it at zero height, actually, one end is slightly off the ground... enough so that when the physics kick in, it drops the whole building at one end and the impact shakes it to pieces.
Was easily solved by using a huge 'smooth' brush in the terrain editor until it would smooth no more. Only caveat is that you'll need to re-import your model again to get the x,y,z translations, because what was zero height is now about -0.39.
SO I'm happy to report I've got stable destructibles now!
