The Rams began well with tries from Farrell, the son of former Leeds forward Anthony, and one apiece from former Rovers wingers Jason Crookes and Etu Uaisele, Shane Grady and Brett Seymour with a goal each.
However, Farrell’s second shifted the game’s momentum with half an hour to play before Matty Wildie got in on the act to put his side 18 points up on the hour.
Though Jack Ormondroyd gave the hosts a glimmer of hope on 65 minutes, Anthony Thackeray sealed a big win for the Rams on the hooter, which leaves Dewsbury just a point adrift of Rovers with two games to play.
With the Broncos fielding nine academy graduates, James Cunningham and Jarrod Sammut traded tries in the opening quarter, but the men from the south soon hit their stride with Rhys Williams, Matt Davis and Joe Keyes all going over for first half tries to produce a 22-6 interval lead.
Sean Morris became the third Londoner to score shortly after the break before Frenchman William Barthau went in.
Town prop Tom Walker briefly stemmed the tide just after the hour mark, but London cut loose in the closing stages as tries from Ben Hellewell, Erjon Dollapi and a second for Cunningham sealed an emphatic win.
Meanwhile, in the the ever-changing face of the Championship‘s relegation battle, Batley Bulldogs eased their fears of a drop into League One with a stunning 50-0 thrashing of Whitehaven in West Yorkshire.
After last week’s heartache against Featherstone, the ‘Dogs were in determined mood from the off, building up a 20-0 advantage at half time after tries from Shaun Squires and Brad Day alongside a double for Alex Brown.
‘Haven, who scored 42 points at Dewsbury just two weeks ago, battled to gain a foothold in the game but it never came as Batley ripped through the Cumbrians at will.
After Luke Blake trundled in on 49 minutes, John Kear’s men were in full flow as winger Brown completed his hat trick before the enigmatic James Brown grabbed a pair of four-pointers either side of tries for Keegan Hirst and Shaun Ainscough.
After failing to win since May, the Hawks have found a new lease of life in recent weeks and recorded their third successive victory on Sunday, overcoming Doncaster 25-16 at the South Leeds Stadium.
Former Dons forward Mark Castle made sure of the perfect start to the second half for Hunslet but Ryan Verlinden’s score set up a tense final 20 minutes.
Jack Lee and Richard Wilkinson then traded tries as the South Yorkshire side threatened an upset, but in-form half back Simon Brown eased nerves in the home camp with a drop goal before a late penalty added extra gloss to the scoreline.