Kevin Gallen, who played the first 30 minutes in preparation for Saturday's trip to Huddersfield, scored the opener after three minutes. Watford replied with two minutes remaining through Swonnell.
Elsewhere, Stuart Wardley has now officially been taken on by Rushden and Diamonds on loan, whilst youngster Justin Cochrane is on trial with Reading after being told by Ian Holloway his contract won't be renewed at the end of the season.