Hendre Lake, St Mellons, Cardiff is probably the best runs water in South Wales. 6 acres, Carp up to mid 20's, average size about 10-12lb mainly commons. Tickets are £6 per day, £30 day syndicate, £60 night syndicate most of the action comes at night so its well worth going on the night syndicate. If you have any more questions just ask.
Others in the area are:
Duffryn Springs, Cardiff
http://www.dyffrynsprings.co.uk/home/index.html
Cefn Mably, Newport
http://www.cefnmablylakes.co.uk/
Peterstone Coarse Fisheries, Newport
http://www.freewebs.com/peterstonefishery/index.htm
Hazel Court Ponds, Cowbridge
http://www.hazelcourtponds.co.uk/
This link shows what I caught from Hendre Lake from May - November 2008
http://www.carp.com/carp-forum/viewtopic.php?t=34825&start=0