There appear to be 2 Pentonville tunes. This one
http://www.hymnary.org/tune/pentonville_linley
and this one

http://www.omniglot.com/songs/bcc/shepherdswatched.php


However... the second tune is called Lyngham in the Hymnary and agrees with what I had heard in the past. So when did the name Pentonville become attached to Lyngham? Is this misidentification by folkie researchers or is it the name given by the carollers?