Hi Dan, thanks for the response.
Sorry, I should have been a bit clearer in my original statement. In R21, the simulation of the bouncing spheres plays fine, just as it does in R20, the problem appears to be the plane effector not using Cloner.1's point information being stored in the mograph cache.
If it helps, the individual that contacted me has sent a link to the video he was trying to mimic. I'll post it here in the hope it offers some further insight.