There was little to choose between the sides in the npower League Two clash until Tom Pope pounced on a rebound to open the scoring on 30 minutes.
From then on, United were always second best and goals from Ashley Vincent and Sam Morsy early in the second half made the game safe.
The visitors might have fallen further adrift in a one-sided final half-hour in which Jennison Myrie-Williams ran riot.
U’s boss Chris Wilder had no complaints over the result and was disappointed with the application of his side once they fell behind.
He said: “As soon as the first goal went in we were up against it and then we conceded a really poor second goal.
“Then really there was only one winner, it was men versus boys from our point of view.”
The heavy defeat ended a run of three games without defeat for United.
Wilder admitted most of the good work had been undone over the course of the 90 minutes.
He said: “It’s a backwards step which is disappointing, because we’ve had a decent ten days.
“I thought we’d turned the corner a little bit, but I think you saw the reaction as soon as the second one went in – it was heads down and people were looking after themselves.
“It really was comfortable for a team in form.”
Peter Leven was due to start the game, but was a late withdrawal after his knee swelled up following training on Sunday.
It is thought the Scot will have recovered in time for Saturday’s home game against Accrington Stanley.