OK - it seems in the end that maybe I owe Nabble Admin an appology ;) I found out that what I had to do was change the 'embedding settings' to redirect new forum sign-ups to the right page. I never thought of looking at the embedding settings, because it never occured to me that that was what I had to look at. I thought that the embeding options was to do with how the forum was embedded (the orientation, how it looked, etc), not where it would be embedded.
So, once again, my appologies. I sorted the re-direct problem