A drop goal from Barry Eaton preserved Widnes' perfect away record stretching back more than a year and took them above the Broncos in the Super League table.
A game of great enterprise and fluency came down to the difference between Dennis Moran's attempt that skimmed the outside of a post and Eaton's successful shot a couple of minutes later.
London went the closer to an early try through Tony Martin and Moran, the latter having one disallowed for a knock-on before touching down. After 18 minutes, they did claim the first try, Michael Gillett kicking cleverly for the post and the big front-rower, Scott Cram, following up to touch down. It was the start of an important first-half contribution from the Broncos' props, but before that Widnes threatened to take control of the game.
Richie Barnett surprised everybody by dropping Eaton's high kick, Chris Percival was alert enough to pick up the loose ball and Daniel Frame had a clear run to the line for a try which Eaton converted to level the scores.
Adam Hughes was the architect of the try that put the visitors ahead. He showed nimble footwork down the left touchline to get away from three tacklers and slip the inside pass to Stuart Spruce who went through to score.
Twice in the seven minutes before half-time, it was the Broncos' prop, Francis Stevenson, who demonstrated the art of try-scoring – something for which he has not been noted.
First he capitalised on Russell Bawden's drive to lunge over and then he took a pass from Barnett to spin out of Hughes' tackle to touch down again. With Martin kicking both goals, London reached half-way in a fluctuating contest with their noses in front.
Links between these two clubs go back a long way, to the early years of the London operation when it was known as Widnes-on-Thames, although the last time Widnes played a London side it was still based at Fulham, back in 1984. If this meeting carried a sense of history, it also had relevance today, as these are the sides likely to be battling for the last play-off place at season's end.
London had a pre-match setback when captain Jason Hetherington was ruled out with a hamstring injury, but this was still the sort of match they need to win if they are to fulfil their potential.
Widnes levelled again when Spruce got a short pass away for Hughes and Eaton landed the goal from the touchline.
As the sides sparred for advantage, Moran narrowly missed a drop goal, before Eaton slotted one neatly between the posts. There were still 25 minutes to play but with London now forced to chase, too many passes went to ground and Widnes remained indomitable on their travels.
London: Barnett; Roy, Martin, Wilkinson, Gillett; Moran, Cram; Retchless, Marshall, Evans, Toshack, Dimmock. Substitutes used: Stephenson, Purdham, Bawden, Moore.
Widnes: Spruce, Munro, Potter, Hughes, Percival, Weston, Eaton, Relf, Cantillon, McDonald, McCurrie, Farrell, Frame. Substitutes used: O'Neill, Stone, Atcheson, Mills.
Referee: R Connolly (Wigan).Reuse content