The 'suspiciously-like a Dusky Warbler' finally got confirmed at lunchtime so I maanged to leave work early and head up there in the afternoon. It was calling when I first got there but flew along the canalside bushes and disappeared. It then showed briefly to RJW before vanishing again. We all split up to search, four of us popping in to Tottenham Marshes to look for it but no sign there. I decided to check from the reservoir side and as I got there I heard it call and could see it on the top of the concrete wall. I called the nearest people over and it started to shoew again although a little bit more distantly. Eventually most people got good enough views of this first for London! A very successful twitch although I couldn't manage to find the Slav Grebe on my way out!