"- Is there anyway of making the leaves look less flat...with like a bump map equivalent in AE?"
You can try to use Free Form to displace the leaf particle (in a precomp) but as Particular will rotate that in 3d space you will still notice that it is a flat plane. One thing you may want to do is to animate the leaf displaced in 3d space and distorted with Free Form in a comp that will become an animated particle and then use that in Particular making sure the particles face the camera always.
"- How do I stop leaves from vanishing? I've tried upping the particle life and upping the distant vanishing?"
Make sure that not only the life of the particle is longer than your clip, but that the opacity over life is constant. Depending on your comp setup, you may have to play with the visibility as well.
"- How do I make the emitter stop emitting after a few seconds. I want the leaves to blow a cross my scene...at the minute its a continuous train of leaves flowing by??"
set the particle number emitted to however many particles you need at the first frame and set the next frame to 0.