My problems with the lambert and reflective shaders were largely due to my lack of understanding. I stopped to think about it for a while and I think I've made sense of the settings.
Struggled for a little with my colour map adjustment node before giving up on doing all of the adjustments to the image in TG and producing an initial modified image that can be tweaked later in TG. In most of the images that I didn't like, there were parts of the leaves that were too dark and too desaturated. To avoid this I calculated the minimum "apply colour" value for the leaf colour using a surface shader (set base colour, play with apply colour slider, read number at the darkest colour), and multiplied that by 255 to give a minimum pixel value for the greyscale image used (12).
I'm pretty happy with the result (larger render attached or
full render here ). My main concerns now are just related to the leaf surface being flat, but there's nothing I can do about that with this model. You'll notice that some of the leaves at the bottom of the canopy are reflecting the sky (which is good) and the whole leaf has a uniform reflection applied across it (which looks odd).
The leaves in the top right best show the result of the shading from using the grayscale leaf image as a colour function.
Here's a screengrab of my basic leaf node setup.
[attachthumb=#1]
Now all I have to do is add in an autumn leaf shader for animation purposes (just a duplicate of the Base colour node and child surfaces with different colours and fractal seeds)... and then tweak the displacement on the bark.