I went to Manton Bay last year and used my 100-400 on a 7D and then attached a 1.4 X to it but still struggled.The nest on the opposite side of the water is 270 metres from the hides and the osprey parents tend to stay over that side. A birder next to me with a Sigma 600mm and an extender did get a decent view but only just.
http://www.ospreys.org.uk/webcam/
http://www.ospreys.org.uk/webcam/