What proved to be Wood's only real chance of the first half came in the 18th minute when Neil Mouat shot over from close range after a corner caused problems in United's defence. Hercules went close again on the hour when his header went agonisingly over the bar after Kevin James' free kick before United scored in the 71st minute.
Wood pushed 9 men up for a free kick which the Ducks' defence cleared to Hercules, he sent a perfect ball to Harthill (who was just about in his own half when the ball was played) and while the hosts appealed in vain for an offside flag which never came, the winger ran on before calmly rounding the keeper Tony Tilbrook and slotting home.
The Wood bench erupted in fury at the decision (hower marginal) but the goal stood. They might have snatched a draw with 5 minutes left but Steed Miles' fierce shot was saved by United keeper Keith Baker and the visitors held on to earn an away tie at Yeading in the next round.