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Dartford pushed Grimsby hard on Tuesday night but conceded two late goals and returned home nursing a heavy defeat.
Striker Jason Prior failed a fitness test on an ankle injury and didn’t travel with the squad, meaning the Darts could only name four substitutes.
Lenell John-Lewis fired an early effort just past the post for Grimsby and
Tony Burman had to make an early change, with Lee Noble replacing the injured Tyrone Sterling.
However, it was the visitors who opened the scoring on 24 minutes. Danny Harris got in behind Paul Bignot and slipped a low shot beyond keeper James McKeown.
Kenny Clark had a point-blank volley saved by McKeown as the Darts pressed for a second goal.
But their lead lasted only eight minutes. Ross Hannah set up Paddy McLaughlin, who fired an unstoppable shot into the net from 25 yards.
Shaun Pearson then headed Town in front from a McLaughlin corner nine minutes before the interval.
And the hosts went into the break with a 3-1 lead after Scott Neilson teed up John-Lewis, whose header looped over Alan Julian and into the net.
The deficit was halved on 55 minutes when Noble took a quick free-kick which caught out McKeown and floated into the unguarded side of the goal.
However, Scott Kerr and Craig Disley scored in the last five minutes to put the result beyond doubt.