This is a case of the missing ZSphere. After initially appending a ZSphere to my high poly mesh subtool and doing some topology editing, suddenly all the history was lost and the low poly ZSphere mesh was no longer editable in Topology; nor I could I bind the mesh in Rigging. I did append a new ZSphere to the original high poly mesh, however the same problem reoccurred again. Any solutions would be welcome.
Silly me. I may have fixed my problem simply by tapping Preview Adaptive Skin off and, voila! the errant ZSphere returned and is now editable.