Thanks very much for the prompt response Matt.
I am using Terragen 3 64bit build 3.1.02.0.
Rendering at a modest 1400x1100, maximum threads 64 but even at this 400x267 resolution the render fails. See attached image.
However when I pull the camera back so that the foreground is not in such close proximity the render works fine. This seems to be the key factor.
I am happy to send the tgd file by email if it can't easily be resolved.
On your other question; Keswick is a beautiful place, you were lucky to have the opportunity to live there. How could you leave!?
I've been using data from
http://viewfinderpanoramas.org/Coverage%20map%20viewfinderpanoramas_org3.htm . I think it's at 3 arc second resolution, so not ideal. I have a fledgling project using LIDAR data, much better but such tiny terrain tiles that need stitching together and everything needs re-scaling, I've shelved it for now.