Northampton Town join MK Dons in League One
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Callum Morton doubled the advantage on 31 minutes, and Cobblers should have been high and dry before half-time but for some solo heroics from Exeter keeper Jonny Maxted keeping the scoreline civil at the break.
But when Dean Moxey was sent off for a late challenge on Watson, the writing was on the wall for Exeter as they slumped to their third play-off final defeat in four seasons. Sam Hoskins made it 3-0 with 10 minutes remaining before substitute Andy Williams stamped Northampton's authority on the game with a fourth.
It means Northampton join Swindon, Crewe and Plymouth in League One next season.