The Cobblers made the perfect start against the play-off contenders with Guttridge's first minute goal and although the U's equalised through Montano, the upturn in fortunes engineered by Aidy Boothroyd was continued with Brett Williams' injury-time strike.
Town still have a game in hand on all of the teams below them in the league and, although they face a tough trip to Swindon on Easter Monday, safety is all but assured.
Prior to the game Boothroyd had said: "We have recently proved that we can win in a number of ways," and was proved right as his side kept going until the final whistle despite some heavy pressure from the visitors.
A bumper crowd of 6,860 were at Sixfields, the majority of whom went home very happy.
Reaction to follow on Cobblers Mad.