http://www.wwt.org.uk/visit-us/slimbridge/

I think out of the bird sanctuary's and centres around here this one from Slimbridge looks to be the most promising. It's website says that there are 13 hides around the area including over the lake and a specific Kingfisher one, which would be good because I've never seen a kingfisher.

Cheltenham to Cam Dursley - £3.70 off peak return (with rail card) then 4 miles to the place, but I could bike that. But that does involve going across the M5 and some tricky looking roundabouts, I'm not sure how confident I am with doing that. The issue with getting to these places are that I can't drive and there never seems to be a bus or train sation that gets me to walking distance. I suppose I could walk the 4 miles, but it still involves getting across busy looking roads.