He signed for Watford in a deal worth £450,000 after playing 27 games, and scoring nine goals for the Kent side after he returned from his loan at Swindon last September.
Whilst at Swindon he scored 5 goals in 6 starts including two goals against Bristol Ciity in his first game in a Swindon shirt. Swindon had agreed to sign him for £40,000 which was accepted by Gillingham, but the deal didn't go ahead as Henderson's wage demands were far too high.