I'm not sure where they are best viewed from Anna but if you check out google maps for Stoney Middlton you will see a road left off the A623 called Middleton Lane leading to Thunderpit Lane....its a steep hill! You will see lagoons at the top marked on the map. Not sure where the best viewing is as I haven't been up for this roost. But if you get there early you should be able to judge a good view point hopefully as they start to arrive?
From what I know the roost is viewed from the hide on Thunderpit Lane itself ie not the main lagoons. If you drive down the lane there is a gate on the right marked no access, this is however the access point. I have never been to this hide but apparently just walk up the hill and it is somewhere over there. Ah I see Steph has sent you up the Lane from Stoney Middleton, in that case the gate will be on your left when you have passed the mill.