Hey, I've just taken this photograph and converted it to a smaller JPEG to show you what I mean.
https://dl.dropbox.com/u/49730000/ZU1S0236.jpg
In the top left and right corners you can see that there are reflections of trees… Can someone tell me how I might be able to avoid those in future?
I am currently using a Gradiented ND Filter for my camera.
I doubt that is vignetting. I've never seen vignetting which looks exactly like tree branches… If it is Cokin-style Gradiented ND filter, it does sit some distance off the front lens and would probably reflect things that were behind the camera. Obviously covering the gap between the lens and the filter with something like a dark cloth would prevent the reflections.
What you see in the corners are not reflections but rather vignetting caused by the filters/lens/lens hood. You can see that it is also vignetting in the bottom corners as well.
If you have stacked a number of filters on, try removing some and it should reduce it. Otherwise, zoom in a bit to avoid the vignetting.