Any clue??

While out birding today in Pilanesberg, I stopped off at Bathlako Dam in the west of the Park. I was then presented with views of the following bird, confusing me no end as I only had one field guide with me. It was considerably larger than the numerous Green-backed Heron moving around, with the body easily twice the size of these Herons.

I have searched through all my books, and I think I have it confirmed. However, I would like to see what everyone else thinks .............