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Frannie Collin came off the bench to clinch victory as Maidstone moved three points clear in the Ryman League Premier Division on Saturday afternoon.
Collin was left out of the starting line-up as Stones went with new signing Billy Bricknell alongside Nathan Elder in attack.
But the substitute made sure of the points by adding United's late second goal after Steve Watt had given them a half-time lead.
Bricknell fed off Elder knockdowns a couple of times, including sending an early effort wide.
But struggling Hornchurch came to have a go with decent chances falling to George Purcell and Thomas Gogo, while Tom Mills saved his side with two perfectly-timed tackles.
Matt Bodkin saw a strike with the outside of his foot pawed away by Sam Mott after a neat move.
Maidstone grew into the game and went in front on 37 minutes with Watt finishing the loose ball after Elder's header from a Ben Greenhalgh free-kick was saved.
Kieran Bishop blew a golden chance for the visitors after a mistake by Sonny Miles early in the second half left him with only Lee Worgan to beat.
In a half of few opportunities, Mott got his hand to Bodkin's attempted lob then denied Bricknell a debut goal with an excellent save.
A United counter-attack led to the second goal with Jack Parkinson feeding Bricknell whose strike was fumbled by Mott, gifting Collin an easy finish in the 88th minute.
Maidstone: Worgan, Huke, Mills, Watt, Miles, Rogers, Bodkin (Akrofi 89mins), Parkinson, Elder (Collin 78mins), Bricknell, Greenhalgh (Brown 76mins). Subs not used: Starkey, Godmon.
Attendance: 2,086.