Hopefully i can help..
Was the car actually loosing coolant at all when you where checking/refilling?
What engine exactly/fuel type does the car have?
When the pump siezed/cambelt...was this during driving or when it was in service?
Did they show you the broken pump/belt or the fault as it was on the car by the mechanic before it was so called written off? and why did the mechanic write it off and not you?
I see ok.. yes the 2.0cdti is unknown for water pump problems..but it normally shouldnt cause code 3 to flag up on the message screen..as you know code 3 is due to level as far as the coolant reservior is concerned..there is small sensor on the tank/base that monitors the level..if the engines loosing coolant then this can cause the code 3 to flag up..so it possible the sensor "could" of been acting up at that time,but leaks would of been part of that check.. but as far as the water pump goes..being mechanical/timing belt driven pump..when faulty they can leak coolant "hence why i ask about leaks surrounding why you had to refill" or they produce a sort of pre failure noise of a cylical scrapping noise from behind the timing cover or around the engine mount area on that side of the engine which is very common to hear when the pumps on the way out..of course to effecting engine temp can also be contributery factor..