The Reds suffered an early blow when centre-back Will Fish was sent off in the seventh minute, for what was deemed to be a professional foul on Ayo Tamiwomo.
However, Neil Wood's side adapted well to being down to 10 men and almost scored when Tamiwomo's defensive header from Shola Shoretire's wind-assisted free-kick looped on to his own crossbar.
At the other end, United keeper Matej Kovar had to make several decent saves in the first half, to deny the likes of left-back James Furlong and striker Teddy Jenks.
United coach Neil Wood freshened up his ten men for the final half-hour, by bringing on Spanish left-back Alvaro Fernandez and Dutch forward Dillon Hoogwerf, and the latter immediately went close to scoring an opener, when his shot from the edge of the area was saved.