Thanks for the response Ed, for a while there I didn't think anybody was listening!
I think I will use the P40 filler over the top of the POR15 as the instructions state. If I believe the attached info that comes with the product, it states that the chemists have formulated the two products to bond together chemically rather than mechanically. Another thing to think about is that there will be more chance of trapping moisture underneath the filler if it goes on after the tie coat, (moisture is our enemy).
I have found a good article on fillers, primers etc' if any body wants to read it. It is written by a pretty cluey guy, definatley worth a read for anybody considering their own bodywork.
http://www.roadsters.com/filler/Wish me luck!!
Damon.