The Welsh international fullback scored the crucial points last night when his strength took him through the middle of the London defence six minutes into the second half to give Saints a lead they had not held since the early stages.
When Tommy Martyn charged down Glen Air's kick and booted ahead to score from what looked a dubious touchdown, Saints looked reasonably secure for the first time and Atcheson's hat-trick try, after Martyn's high kick had bounced back off Dominic Peters, confirmed that the tide had turned.
Anthony Sullivan's second try, after Greg Fleming had knocked on 20 yards out from the Saints line on the first tackle, took them clear.
This had looked anything but a simple assignment for the Super League leaders in the first half. They struck first, when the first of Martyn's succession of towering bombs had bounced between Tulsen Tollett and Martin Offiah for Atcheson to grab his first try, but the rest of the first half saw London in the ascendant.
Fittingly, it was their former St Helens stand-off, Karle Hammond, who did much of the damage. His magnificent kick for the wing picked out Brett Warton for one try and, after Peter Gill had dummied his way over for another, he created an opening for Fleming in superb style.
Taking a difficult low pass, Hammond, whose decision to move south is of great symbolic as well as practical importance to the club, gave a cultured inside pass to send Fleming over, a second goal from Warton giving the Broncos a 10-point lead. Unfortunately for Warton he later broke a bone in his arm and will miss the Challenge Cup final on 1 May.
Saints made inroads into the lead when Martyn's long pass sent Sullivan in for his first, but the winger's disorientation after a hard tackle gave Warton another goal when he played the ball in the wrong direction early in the second half.
The second half belonged conclusively to Saints and they were safe well before Tim Jonkers went over for a try to round it off in the last minute.
London Broncos: Tollett; Warton, Fleming, Timu, Offiah; Hammond, Air; Retchless, Beazley, Seibold, Simpson, Millard, Gill. Substitutes used: Ryan, Toshack, Callaway, Peters.
St Helens: Atcheson; Tuilagi, Iro, Newlove, Sullivan; Martyn, Long, O'Neill, Cunningham, Davidson, Nickle, Perelini, Sculthorpe. Substitutes used: Wellens, Jonkers, Smith, Joynt.
Referee: R Connolly (Wigan).Reuse content