Thanks for the suggestion, JL - unfortunately that 'fit' option won't work in this situation.
What I have is a large topo/terra model which extends a number of scale miles beyond the plant proper data - and the texture map I have at 5Kx5K still falls short of covering the entire 3D model.
I don't have access to the original 3D model data, using a larger texture map crashes my system, and using a map of smaller resolution isn't an option as the renders and ultimate fly-by will be of fairly close positions.
Oh well - that'll teach me.
Thanks again for the advise - much appreciated.