SAINTS skipper Paul Wellens struck at the double to move his side nearer a top four play-off spot.
Mark Flanagan also crossed twice for Saints who led only 6-0 until just before the break.
The Broncos were in confident mood after three wins in a row - the last a 62-18 hammering of cup winners Warrington a fortnight ago.
London were missing Chris Bailey, Chris Melling and Michael Witt, with Dan Sarginson still out after a month’s absence with an ankle problem.
Saints welcomed back Jon Wilkin and Josh Jones to compensate for the loss of Michael Shenton and Chris Flannery.
Saints, who won 34-24 at the Stoop on the opening day of the season, took only three minutes to go ahead when Tony Puletua burst on to a James Roby pass to score near the posts and Jonny Lomax converted.
Saints took advantage of a high tackle by Tony Clubb on Roby in the build-up to Puletua’s opener.
Clubb made amends by intercepting a Lomax pass to deny Saints a second try and Broncos hit back to apply pressure.
Referee Richard Silverwood awarded 10 penalties in a stop-start opening 17 minutes.
Saints introduced Louie McCarthy-Scarsbrook off the bench midway through the first half and he immediately made his presence felt against his old club with some tough tackling.
A storming burst by Sol Soliola tested the Broncos defence, which was equal to the task until another penalty allowed Lomax to carve out a try for Mark Flanagan after 35 minutes.
Lomax missed the straightforward conversion to leave Saints ahead 10-0 with Paul Wellens having a try disallowed just before the break for a forward pass.
Saints made a storming start to the second half with Luke Dorn, making his 200th appearance in the British game, forced to race back to pull off a try-saving tackle on Francis Meli.
But a blunder by Omari Caro, who failed to collect a Lomax kick, led to Flanagan’s second try after 45 minutes to make it 14-0.
Then Wellens added Saints’ fourth touchdown to score for his fifth successive match but Lomax missed his third conversion from four attempts.
Clubb was held up over the line for the Broncos before Wellens took Puletua’s pass to grab his second try and make it 24-0 on the hour.
Dorn almost notched a Broncos try in front of the 13,262 crowd but was recalled for a forward pass from Chad Randall.
London held out until Anthony Laffranchi crashed over for the final try in the last minute.