Harrop cross from the right, was well saved by Isted.
Saints grabbed a lifeline on 67 minutes when substitute Barnes slipped the ball past Willock and drilled a low drive past Henderson from 25 yards.
Mitchell and Barnes had good efforts thereafter for United and Saints respectively, before Gallagher missed a great opportunity to level proceedings on 76 minutes after failing to divert Nehemie’s cross goalwards from five yards out.
Despite further late pressure from the visitors, the Reds held out for the win, their sixth on the trot.
United: Henderson; Evans (Goss 46), McConnell, Borthwick-Jackson, Barber; Riley, Harrop, Willock, Mitchell; Croskery Redmond 60), Fletcher. Subs not used: Thompson, Pereira.