There are 3 sets for each exam. They are held by a ground school examiner who is responsable for them and in theory they should never leave his sight unless secure.
Every so often (about every three years) they rewrite them and issue a new set to each examiner they then usually have to again after a couple of months because there will be that many complaints about certain questions.
I suspect what you would call a bank is actually a lump of A4 dumped in the back of a filing cabinet which see's day light every three years for a week.