Sorry, Sooty
It isn't a case of either-or. It is and-and-and.
My (amateur) analysis of water source candidates based on information available is:
3Kg Max dissolved moisture at Beijing uplift (AIB all tanks)
2Kg Max entrained moisture at Beijing uplift (AIB all tanks)
Max adiabatic ingestion into centre tank (single air mass assumed):
0.4Kg descent to Beijing 0 degrees @ surface = 4.8 mg/litre
1.2Kg descent to LHR 15 degrees @ surface = 12.8mg/litre
Max adiabatic ingestion into main tanks (total for both tanks)
0.2Kg Beijing
0.7Kg LHR
Plus an unknown amount caused by centre tank fuel purge jet pumps sucking air for two and a bit hours after the tank is emptied (this may be the AIB's 0.14ltr (=0.14Kg) drawn in through the fuel tank vent system)
Plus any remnants of dissolved and entrained LHR uplift
Plus trapped water that cannot get to the sumps (0.35 litres total found in the main tanks at the earlier inspection - AIB) which is almost certainly totally innocuous.
The total is something like 8 litres max.
About a quarter of the uplift would start in the centre tank, so the centre tank could contribute some 2 litres or more. The centre tank is IMHO significant because instead of sucking air the fuel purge pumps will inject its water into the cold main tanks once the centre tank warms up above freezing.