OK, this isn't exactly drawing a flood of replies, and I guess I can understand why. It's probably hard to follow since I've worked out the problem from first principles rather than using physics terms and formulae. So let me simplify the question, and this might help me work out what I need to...
I have written a computer game which involves 2D asteroids spinning and colliding with one another. Out of some kind of pride or something I wrote my own physics engine to handle the collisions, even though I never studied physics. (Please excuse therefore any incorrect terminology)...