Henry,
Here's something really basic I just whipped up that simply moves layers away from a null. It just pushes objects away from where the null is when they're closer than the "Distance Threshold" control. You can increase the amount that they move with "Move Away Multiplier."
Just paste the position expression from one of the solids to your own layers and it should work or at least be a starting point to tinker further. Expression pasted below as well:
Here's a quick screencapture showing what it does:
And here's the .AEP:
Not fancy but maybe helpful?
 - Nathan
p = transform.position;
nullPos = thisComp.layer("Parent And Controls").transform.position;
distanceThreshold = thisComp.layer("Parent And Controls").effect("Distance Threshold")("Slider");
moveAwayMult = thisComp.layer("Parent And Controls").effect("Move Away Muliplier")("Slider");
distance = length(p, nullPos);
if (distance < distanceThreshold) {
  distanceMult = (Math.log(distanceThreshold / distance)) * moveAwayMult;
  delta = nullPos - p;
  nDelta = (normalize(delta));
  newPos = p - nDelta * distanceMult;
} else {
  p
}