Bittern at Erddig?

Last weekend, we were out at Erddig, in North Wales, and I had forgotten to bring my binoculars. 

What I initially thought was a buzzard seemed to get flushed out from general direction of the lake in the main gardens, although I didn't see where exactly. 

What caught my attention was the flight pattern, which seemed more... agile(?) than a buzzard, and then when I looked more closely, the throat seemed creamy white, and the legs trailed behind. 

Thoughts on what this was? I asked the NT volunteers at the estate, and nobody I spoke to was aware what a bittern was, let alone if anybody had spotted one.