In Spring this year we had to wait at LGW while a Pully 747 taxied out. My FO spotted an even larger fuel spillage than that shown here from the Rt wingtip. Told ATC and it took a bit of convincing to get the Pully crew to get ground assistance to check the leak.
Finally they had to taxy back in. You could smell the fumes through the Aircon from fifty meters.