Thank you Arsheesh
Yes, looking at it fresh today after sleeping on it I can see what you mean. I might update it once I've finished reinstalling Windows.
Blender isn't the easiest thing to learn, but if you do decide to go down that route the trees are generated by a plug in called something like 'create sapling', or 'create tree', which you have to switch on yourself in the software. Then you have to arrange your tree with the options and texture the leaves and the bark. The lighting needs arranging so that you have 2 sun lamps - one on either side looking down at the tree at about 45 degrees. One of those sun lamps has to be very dim since it will act as backlighting in the otherwise totally black shadows. The camera has to be set up directly above the tree looking down on it, and then the scene rendered from Blender as a png file.
Once you set up a single scene this way, however, and providing you are careful to grow the tree last of all and don't click anything else, you can cycle through different trees by clicking the random seed in the 'create sapling' box.
Once you have about 20 (which seems to be a sufficient number of different trees for a map of this scale), you can open all the PNG files in GIMP and subtly alter the exact shade of green and how light or dark they each are to further increase the visible variation between the individual trees of the set.
And that's all there is to it
They aren't as good as trees made in Vue, or isolated from real photos, but they seem to work well enough for a map.