The Shakers lead from the second minute, and thanks to goals from Billy, Nelson and Lawson were three-nil up at the break.
Two late goals from Smith helped the home side add some respectability to the scoreline, but they proved to be nothing more than consolation goals.
Torquay United's 1-1 draw at Bristol Rover's moves them upto forurth, a point above United.
Elsewhere Southend United recorded a one-nil win at Shrewsbury, while a late equaliser from ex-Iron Richard Hodgson saw Darlington take a point away from Oxford United.
SUFC are next in league action on Saturday, when they travel down to face Bristol Rovers at the memorial Ground.