Re:
http://www.pprune.org/4392955-post1983.html
If refueling was done after sumping at LHR what about the possibility of contaminated fuel being loaded at LRH? Unlikely, but if I understand correctly they had a water alert at LHR? Say that water was correctly scavanged and sent to the engines causing a layer to build up in the plumbing (not enough to cause a problem in itself). The contaminated fuel would have been used up and replaced with good fuel in china. It was cold there so ice in plumbing (if not the tanks) might have remained frozen? Then on way back ice build up reduces until the last bit brakes off in chunks and causes the serious restriction. Result no water detected in the tanks.