The Pilgrims were well in the game before half-time, but once Ben Clarke had handed the visitors the lead moments before the break, United well and truly imploded.
Two goals from Kurtis Turner, plus a second strike from Clarke, in addition to goals from Joe Flint and Ben Birch did the damage.
Boston (4-4-2): Tom Benton; Macauley Britton, Lewis Sturman, Jordan Nuttell, Corey Newman; George Stainfield, Aaron Eyett, Jordan Sowerby, Lewis McKue; Leon Squire, Juris Kormis (Lemos Pereira 46). Subs (not used): Jacob Pease, Charlie Melville.