Key moment in the game came in the third period when Southampton, leading 12-10, lost a player to a violent conduct incident. By the time the four-minute penalty had been played out, Sedgefield were level at 14-14.
To Southampton's credit, they immediately went ahead again - a powerful shot from eight metres by skipper Charlie Roberts, one of his seven goals in an outstanding individual display. But Sedgefield were soon ahead again and, with John Pratt taking his personal goal tally to six, they broke clear in the final stages.
The trophy and medals were presented to skipper Matt Calvert and his team by former international referee Andy Allen.
Invicta beat Hucknall 15-13 to secure second place while the third promotion spot to Division 2 went to Worthing, 16-3 winners against Manchester Hawks.
Southampton, Manchester Hawks, Hucknall, Croydon and Otter go to Division 3 where they will be accompanied by Rotherham Metro (6th team in Division 3 this season) and two from what promises to be very competitive playoff tournament involving Castleford, Cheltenham Spartans, City of Birmingham and Bridgefield.