McHale,
Your helpful input encouraged me to get out the FCOM2, Pat Boone's MRG, Brady's technical guide and Bulfer's book and I found a note in the latter which stated that an inadvertent transfer of fuel into the centre tank can occur if a check valve doesn't seat properly. Not much else in any of those titles to help solve the riddle.
As for figures, I have the following to hand:
Sector 1: Dep 7.8 Arr 4.5.
Sector 2: Dep 6.0 Arr 2.9
Sector 3: Dep 7.8 Arr 4.5
Sector 4: Dep 6.2 Arr 2.8
On our second sector I shut down engine no.2 for the taxy to stand and I do recall that there was a smaller 100-150kg imbalance at that time. The main imbalance occurred on Sector 4. Interestingly, the imbalances seemed to have occurred on similar length sectors.
By the way, the fuel in the centre tank was not evident on arrival. It only appeared after fuelling the aircraft.