Thanks for the input everyone. It's a challenging problem.

Pilou, I like your solution! Actually I had forgtotten the sandbox tools. The reason why it produced strange results the first time (as Michael said) was because of the boundary rectangle at "z=0". Remove the rectangle and SU produces a pretty good result.

The idea of producing a DEM is also a good way to go. There's a neat app called Blackart which does this quite well. Grendel, thanks, I'll check out Zsurf. A 3d surface can also be made by applying the heightmap as a displacement map (blender, etc)

Generally the contours come from pixel images. Can be cleaned up a little (Gimp, etc), vectorised (Blackart or wintopo, etc) and saved as pdf, which MoI imports sweetly. Then join all contours, fix holes, get the scale right, elevate contours to correct heights.

Michael, the extrusion method is quick and could be useful sometimes. I wouldn't be surprised if the resulting mesh could be smoothed in some other app (Blender, maybe)

Anyhoo, it looks like whichever way it's done, it's gonna take time!

Steve