So when does this happen (Rightbase)?
5LY in post 326 had an answer:
The center tank pumps are manually turned off when the tank reaches 2000 pounds (900Kg.). The scavange system then operates automatically to draw out the remaining fuel when the total fuel remaining reaches 29,000 pounds (do your own conversion).
Doing the conversion gives 13,500kg. The aircraft landed with roughly 10,500kg on board (AAIB). So rough calc suggests the scavenge from the Center to BOTH Main tanks started about half an hour before landing (72,000kg/12 hours = 6,000kg/hr of flight). At what point did it reach the possible contamination - depends on rate of flow of scavenge system... - but the contamination reached both main tanks at roughly the same time... thrust reduction 7 seconds apart ... (pure speculation of course!).