[David Horler] “I have tried Particle Playground as it seems to have a good set of 2D physics which is what I need. However, I can’t change the particle sprites to circles (rather they stay as default squares) and I also can’t set their speed to 0 when they enter a certain area (however, I have managed to make them shrink to 0.5 scale when they enter the area as a test).”
Particle playground can use any layer as a particle the setting is under Layer Map. It uses Property Mappers to control size and speed. As Roland wrote, there are plenty of tutorials on the Cow.
Another option is CC Particle World, better still if you have it Trapcode Particular.