Thanks for the comments on the pictures, I really appreciate the feedback.
I've been experimenting with putting an extra light source near the surface of the water and just behind the camera. By varying the strength of light and running a series of low grade tests (320 x 240, 0.5 detail) I've been able to find a level I like. It's a bit of a balancing act trying to get the right results - too light and the trees look too illuminated and unnatural, too dark and it's back to square one.
Problem is, after finding the right balance I've tried rendering at my usual settings (1024 x 768, 0.9 detail) four times and each time have had the dreaded Red GI bug. I'm not really willing to get rid of GI altogether as I've finally found a lighting balance that I'm happy with, so I'll keep trying with my current settings and hopefully at some point it will be a success. Alternatively I'll wait until the red bug is fixed in a future update.
Once again, thanks for the comments.
Inscrutable