Sledge: that [multiple updateworld's] didn't help I'm afraid. Same thing is happening :-(
Sorry - it's a bit more complex and, thinking a on it a little more, not really suited to multiple targets (because it involves translating the colliding objects along with the collided one then subtracting the translated figures from the colliding objects regular movement. I can post if you like though.)
Everyone is going to yell "IT'S SLOW! IT'S SLOW!" but my gut reaction to solving the two-object-types-moving-at-once problem would, in this case, be to check the distance of the bullets to each enemy and if they're very close then go on to see if their meshes intersect.