Yep! I set 7 as the active texture unit, bind the texture,and then set 7 as the value of the gCascadedShadowMap texture uniform.
I was hoping someone might now what possible things could 'block' the texture from being read from? Are their constraints on the bind states of the frame buffer objects? I've tried binding and unbinding the relevant frame buffers in various places, but I'm sure I've not exhausted the possibilities.