Cheers guys, my capx is a bit of an unholy mess at the moment but I'll try to mock up something like what I want to achieve and post it up soon. I did manage to get something sort of working with solids, but as before - and even with the refined collision boxes - they glitched around a lot when they clumped up, and sometimes jumped through walls in the tunnels, which isn't ideal!
Actually I got sort of halfway there with a combination of MoveTo and the Boids flocking behaviours. The mobs chase and keep a good distance from one another without glitching - now I just need to figure out a way to get them to navigate around obstacles.
monitz87 your solution sounds interesting, I'll definitely give that a go too. I must admit half the functions in MoveTo are a bit of a mystery to me, I just use it as a lazy man's shortcut to something I'd usually do with Custom Movement!