Thanks for replying. Here is what I am doing:
I have a scene (on track 1) which I duplicate an identical copy to a track layer above (track 2). On a track above that (track 3) I have a PSD file that has a black orb against a white backdrop. I add a blur and reposition the orb to the correct part of the scene i wish to manipulate. I then add a 'Track Matte Key' to the duplicate scene (on track 2) and tell the 'Track Matte Key' to matte track 3. To finish I apply effects to the duplicate (on track 2) to suit my liking for that part of the scene. Sometimes that seems to work, other times it doesn't. Is there another way to achieve the same result? Perhaps something simpler?