Before I refit the tank, I would want to know exactly where the water was going. You must have a leak somewhere to loose a tankful. Worst case would be the head gasket and the coolant is going into the engine, or dribbling slowly outside the engine and evaporating as its trickling out.
Other two places I would look at is the radiator and the heater matrix, both are prone to leaks when the cars are getting to the age of our TF's
Underfloor coolant pipes are another bet, but most of our cars have had them refitted by now.
You can get a dye that shows up under ultraviolet light, we use it on machines at work when we are trying to find leaks in coolant systems.
Wonder if it could be employed here to help you spot where the coolant is going.
Have a google search for ultraviolet dye and see if there is anything suitable.
As for your coolant expansion tank, they are very robust and I would not imagine that this has failed.
Sundance