dxzh thanks for the info - 2 items that stand out from your post from my point of view are :-
1 - • Excess water maintenance messages are not drawn to the attention of the pilot (unlike say an EICAS advisory or caution say about low pump pressure). The excess water maintenance message seems to self-clear even if correctly triggered.
2.-• It is only designed to be triggered in centre tank if more than 627 litres (138 gallons) or so of free water is settled at bottom of particular part (any idea which?) of centre tank. This is insignificant amount in context of burning 80 tons of fuel but even a part of such amount might represent more than enough water, suitably frozen, to create a significant restriction in each of the fuel manifolds.
Can somebody confirm to me that the two ponts above are absoloutley correct. That is to say that the excess water message will self clear and that approx 138 gallons is the amount of water that could be in the tank prior to any sensor being activated.