Here are a couple of pictures of a Bittern taken at London Wetlands centre (6 miles from the City centre) a couple of weeks ago.
Gives an idea of how well camouflaged they are in the reeds.
When I was younger it was very rare to see these birds. I remember traveling to see them in NW England at Leighton Moss and then the excitement when they first re-appeared in the Lea Valley in Essex. Now we have them in central/west London. A great story of species revival.