Thanks for the reply, I did try what you suggested and didn't really find that it changed things. The problem seems to get worse, the further into the fog the paricles are.
The material applied to the particles is an A&D based material, with a greyscale image placed in the Cutout slot to handle the opacity.
I'm trying to attach a couple of images to show the problem, but it doesnt seem to actually upload them when I click to add attachment...
Maybe I'll just have to try a different method of adding this kind of thing in post. What I'm trying to do is show a subsea scene that is focused on the legs of an offshore platform, where some plantlife has grown on the below water areas of the legs...