Another site wrongly asociated with Beech Bootom Dyke (in St. Albans) and Devils Syke and the Slad (near Wheathampstead). These three defenses are unlikely to have ever been connected to each other.
The dyke is on private land near Mayne Farm on the Gorhambury Estate. Apparently it's 15 metres wide, and is thought to have been conneced with the Catuvellaunian capital at Prae Wood (of a later period to the other sites mentioned above).
I've not visited the site, so I should give credit to James Dyer (Discovering Prehistoric England) and Nicholas Thomas (Guide to Prehistoric England) for this description.