Sorry of this appears twice but my frst attempt appeared to fail as my attachments were too large, however it kept telling me I'm trying to double post when I resubmitted it with smaller images. The post doesn't seemed to have appeared on the forum so I am trying again.
The following 3 images show the preview window, quick render and full render of the same scene using the same camera and no changes between each. Hopefully you can spot the differences between the renders which whilst interesting is not really what I was after. :o
I think it may partialy stem from the second highfield which I was turning into some cliffs but decided I didn't like. I couldn't seem to remove it when I was trying at first. However, when I exited the app and reloaded it later on it never appeared in the preview but is still in the tgd. I don;t remeber the 'cliff' heightfield looking like the full render and I think it filled a slightly different area of the screen, but I may have changed somethings when trying to get rid of it before saving.
The tgd file is attached here aswell.
You've got a fairly large but small scale displacement going on one of your surface layers. Because of the small scale it's only properly visualized at higher detail levels. Certainly an undesirable result, but one that's difficult to avoid.