And also "If so, and water sumping was done correctly, there would be very little water in the tanks. This, together with sumping on 14 january during maintenance should imply the tanks were virtually free of water as G-YMMM departed to Beijing."
All this is qualitative, not quantitative, which made me wonder whether we were talking about 5 ltrs, 20 ltrs or 100 ltrs being "significant" or "virtually free of water"..
I sump fuel tanks on aircraft. We drain out about a litre into a clear plastic container, and you might see a globule at the bottom of the bottle, less than 1cc. Sometimes you find nothing. So a decilitre would be significant, and 5 litres cause for an investigation.
I also work in Sweden, we do not sump tanks when the OAT is below zero. Bit silly really, but ice will not come out!!