Chorley twice came from behind, their ‘character and resilience’ praised after the match by boss Matt Jansen.
In a bright opening spell, Chorley went ahead on 10 minutes. From an inviting Marcus Carver pull-back, Sefton Gonzales beat keeper Dave Carnell at the second attempt.
But six minutes later the visitors equalised in fortunate circumstances. George Thomson whipped over a cross-cum-shot which deceived Sam Ashton in the home goal and found the far top corner of the net.
The referee took the unusual step of signalling a drinks break after 25 minutes.
The loss of momentum perhaps triggered a loss of concentration among the Chorley players. Indecision following a home throw-in led to a badly under-hit back-pass which allowed the alert Jerome Wright to gain possession, round Ashton and place the ball into an empty net.
The Magpies took time to shake off this setback but with a minute to go to half-time they drew level, Dale Whitham teeing up Andy Teague who netted with a crisp low cross-shot.
United started the second half on the front foot and Tom Greaves missed a clear chance, shooting wide.
Then ex-Magpie Sam Sheridan was only just off-target with a 25-yard snap-shot.
Chorley were undone on 63 minutes by a sweetly-struck Thomson free-kick from 20 yards which zipped just inside the post.
It began to look as if United would hold on – until the fuse was lit again in a frenetic last six minutes.
Chorley substitute Jason Walker was bundled over to earn a penalty but the ex-Barrow striker’s shot from the spot was brilliantly parried for a corner by Carnell, sparking wild celebrations among the United players and hundreds of their fans behind the goal.
But the reprieve was short-lived. From Whitham’s well-flighted corner, Gonzales rose above the visitors’ defence to get the vital headed touch which made it 3-3.
Chorley piled forward and had United hanging on but, with players on both sides now fully pumped up, a lunging challenge by Teague in the visitors’ penalty-area earned the Magpies’ skipper a second caution and the inevitable red card.