I also live in Stevenage and have recently been searching for new waters. Here's a few that I have fished and also ones I have found on the net;
Day Ticket Lakes
Chestnut Pool (real runs water) - http://www.chestnutpool.co.uk/uk/home.html
Manor Farm (2 carp lakes are medium difficulty) - http://www.manorfarmfishing.co.uk/
Banjo Lake (Not fished here) - http://www.leevalleypark.org.uk/en/content/cms/leisure/angling/banjo_lake/banjo_lake.aspx
Holwell Hyde (pay at bank - could not find website so this will have to do) - http://www.go-fish.co.uk/venue_2185.htm
Rib Valley lakes (hard water, nice fish apparently) - http://www.ribvalleyfishinglakes.com/
Stanborough (not a fan of this place, nowhere fishes that well and boats are annoying) - http://www.fishsoutheast.co.uk/venueshertsstanborough.htm]
Clubs
None of the following I have fished. These are only clubs that I have found on the net recently.
Gerrards Cross - http://www.gxudas.co.uk/
BADAC - http://www.badac.co.uk/
Verulam - http://www.verulam-angling.co.uk/
Biggleswade and Hitchin - http://www.bighitaa.co.uk/
Blunham Angling - http://blunhamac.co.uk/index.html
Syndicates
CEMEX water - http://www.cemexangling.co.uk/new_site/venueinfo.php?function=viewvenue&venueid=161&groupid=2
Season ticket for Manor Farm - http://www.manorfarmfishing.co.uk/coarse_season_tkts_28.html
I hope this information helps,
Craig