Ok I have simply assigned the matte to a poly plane and modeled the rocks as seperate pieces of geometry. This way it will work a lot better with the previous shots and be easier. I will need to add a camera shake as the rocks break away at the start and there will be another when the character who is in the shot reaches the camera but I am yet to get to that as he is behind the rocks and will only become visable after the large rock passes the camera. I am trying to work out how to go about creating a simulation with dust and smaller debris to come down with the falling rocks 1st though. Also is there a way to create sun beams to shine out around the rocks and dust in this shot.

Thanks for your help and advice with this guys it is much appreciated. I would of liked to created the scene as a simple 3d model and made it identical to the matte by taking it into zbrush but this would of probably taken a good days work and this method should work fine if I can suss how to get these other little tweaks going on.

https://www.youtube.com/watch?v=F_3GFp8Dvo0