when placing track etc on a map generated by dem, is there anyway to stop the minimap from loosing the printed dem or topo map representation and reverting to the standard gray representation?
The original Minimap is created by HOG or TransDEM and these programmes can fill the minimap (it's a TGA texture) the way they like, in the full TGA color range. If you modify a baseboard, e.g. by adding track, Surveyor recreates the minimap from the terrain ground textures and hereby replaces the original. If the ground is not carrying the topo-map texture overlay, the minimap will revert to gray.
If, on the other hand, the original DEM-based terrain is painted with HOG or TransDEM ground textures representing the topo map, these will be found back in the minimap even after recreation - as long as ground texturing proper has not started.