Thanks DandelO, it seems that your advice worked: I created a surface shader, captured it inside a group, (EDIT: it appears this isn't necessary - just connect straight to the shader) and then I was able to assign it in the surface shader tab of the object. Next time i looked, the final inputs box was there, and I was then able to disconnect the surface layer and re-link the multi shader.
Still took me a while after that to figure out that the Luminosity of the default material plugged into shader 1 had to be non-zero for the grass to be anything other than black!
EDIT: @Oshyan - I have no idea what is causing this, don't think it is overlapping nodes