Covenants of Stonehenge
Nigrasaxa is located in Suffolk. The covenant is a fairly young one, founded a mere twenty years ago in 1201, by Maximianus of House Bonisagus.
Stephen Eruditus of Jerbiton founded Libellus covenant ten years ago (a relatively short span by magical reckoning).
Denning of House Quaisitoris founded Lex Hermeticum some fifteen years ago. It is a covenant dedicated to the study of Hermetic and Mundane law. It is located just outside of the city of Winchester.
Much of the background information is derived from the 4th edition Ars Magica Supplement – Heirs of Merlin and various free downloads (specifically wrt Nigrasaxa and Libellus).
From the Ars Magica wiki
From another group’s list of Covenants
An interesting article on mythical medieval Europe:Places that don’t actually exist