Cotswold water park is a well known site - unfortunately not that well known by me. You should be able to get more detailed info on this from other birders - but be careful! Some part that people used to visit has been declared a conservation area by the new water park trust, so some areas people used to wander into they should really respect the signs and keep clear of (there is working machinery in some areas, anyway, so there are good practical reasons to keep to the marked areas).
There is nothing there yet, but there are plans by the local RSPB group to put more local info on their website at
http://www.communigate.co.uk/wilts/nwiltsrspb/page3.phtml once they get it organised - but they've only just asked members for info so don't hold your breath.
I know nothing about them, but there is a Wiltshire Ornithological Society that meets in Devizes. I have a flyer from them because I bought the 2002 Wiltshire Bird report, which came out a month or so ago. If you PM me I'll see if I can find an address for you - I don't know if they are on the web yet.