I used to belong to
Welwyn Garden City AC who have a half mile stretch along by the Garden Centre on the B158 at Bayfordbury just south of Hertford.
It has a good stock of Barbel and you can fish at night there too. We only used to catch small barbel between 3lb and 7lb but the NRA netted a 13lb Barbel there a few years ago; and the average size may have increased since we used to fish it.
It has an awful lot of streamer weed though. the most productive swims seemed to be at the upstream end in pegs 1 to 5 but there were also some very good swims at the other end near the railway viaduct.
I have heard that the famous ‘Red Spinners’ also have a section not far from there but of course they keep what they catch fairly quiet.