Thanks, these gave me some ideas. The difference between the quick and full raytrace is 1s to 3minutes and does fix the problem with a significant hit.

I've only got Adobe Premiere Elements and it's not capable of the compositing effects described as far as I am aware.

Thanks to the comments I did have an alternative idea. If I render the 2 background objects with Depth of Field and then use the resulting image as a background, rendering with those background objects invisible then I should be able to get the best of both worlds.