Castleshaw Reservoirs was included in a county gull watching article on the Manchester Birding website which can be found in the 'articles, reviews and identification issues' page for those who haven't seen it and covered just this subject.
I agree with Steve's comments regarding the gulls at Castleshaw. I live on the flight path between Castleshaw and Audenshaw.About an hour before dusk there is a constant passage of gulls flying towards Audenshaw from Castleshaw and the area around Strinesdale res.
As organiser of the last Winter Gull Survey, I'm pretty sure that gulls don't actually roost at Castleshaw. Plenty can be seen loafing there during the day but I've made plenty of visits at dusk and they all move off down the valley - presumably to Audenshaw.
Visited this area of Oldham this afternoon primarily for a walk but was pleased that I had my binoculars when I unexpectedly disturbed 2 Crossbills from a conifer near Grains Bar. I suppose these birds might be from those seen at Dovestones earlier in the winter? - which isn't too far away as the finch flies!
Looked across to the eyesore that is the landfill site of High Moor Quarries at Scouthead. This site (understandably) isn't too popular with the local residents but it seemed very attractive to good numbers of gulls and corvids. All three species of large gull were present - I presume these birds roost at Castleshaw?
I suppose these birds might be from those seen at Dovestones earlier in the winter? - which isn't too far away as the finch flies!