"Duncan Weir - what an outstanding prospect, the way he controlled the game - he can kick the ball, " Lineen said of the player signed from Glasgow Hawks.
Centre Hape then touched down before Henderson plucked the ball out of the air from Harris' perfectly-judged kick to put the game beyond London's reach.
Paul Hartley tried to haul the home side back into the game, but goalkeeper Graeme Smith beat out his free-kick and dived on the loose ball with several attackers waiting to pounce.