Thanks for the interest. It's strange, the large scene mentioned above renderes fine at a lower resolution ie: 4096 X 3072 but not above.
A very basic scene (sky and ground) can render at any resolution without problems.
Steve at E-On still thinks that it is memory related and he might well be right but I have 2GB ram - how much does it need?
The only other point that might be giving trouble is the scenes that won't render at high resolution have one thing in common, they all had a poser model imported? - could this be where the problem lies - I need to do some more tests.
Thanks for reading.