A 'fully' flat battery can be ruined
My Mrs did the same, left lights on, on our new car (Metro), years ago, and even after a full trickle charge and 100 mile trip it still lost charge overnight
It could have been a faulty battery from the offset but the AA mechanic, and I forget how, opened all the top-up holes and one was 'bubbling" and he said that cell was dragging the others down