I decided not to walk around, but instead stood on the dam wall by the middle reservoir for about half an hour, to see what I could hear! Surprisingly, in that short space of time I heard, but didn't see, the following species: Kingfisher, Wren, Grey Heron, Bullfinch, Chaffinch, Blue and Great Tit, Nuthatch, Grey Wagtail and Mandarin Duck.
As I was leaving, the fog had lifted enough for me to make out a few species, including 9 Black-headed Gulls (made up of 7 adults and a couple of 1st winter birds, like this one), half a dozen Mallards, 2 Coots and a Grey Wagtail that was feeding in the overflow channel. Not too bad, for a very misty start.
Black-headed Gull (1st winter)