Weight map killing other Landscape layers!

So I have built an Auto Material as per the one here: Create Massive World Landscape Auto Material thought I would give it a try because it included a water layer, which I hadn’t seen before in an auto-material, and its great. Anyway I wanted to get the best of both worlds and integrate some weight maps to set specific boundaries for say crops and forest areas which don’t follow the slope oriented construct of the auto material.


Anyway the DeciduousForest texture ends up where it is supposed to, the Alpha using grayscale weight map to provide the distribution, but the rest of the stuff, the auto-material blanks out. If I reverse the connectors to the pins, i.e auto-material to TopMaterial and forest alpha to Base, then as you might expect the AM becomes visible and the forest blanks out. I suspect I have miss-understood how a texture mask works, and that everything not in the white will be masked out. So how can I provide this delimiter to my forest layer without mucking up the AM?
Originally I expected to shuv it into a LayerBlend with the AM, and import the mask through Target Layers as I would have don’t if the Landscape was purely built with weight maps, but whilst the imports went ok, the layers never appeared in the Layer Parameters of the Material Instance, so I couldn’t assign the textures.

Anybody got any ideas on this. Thanks.