that you might be over-reacting just a tiny bit?
This is obviously an error, albeit a stupid one, and errors occur every day and in every walk of life. So far no harm has been done, and the chances of any unauthorised third party getting your card number are remote in the extreme. Sending something over the Internet does not mean that it's there for "virtually anybody to intercept and see", in fact it's extremely difficult to intercept email en route.
There's no need to cancel your card - you'll be totally covered in the unlikely event of any fraudulent use, and as for one of the bank's 'legal reps' phoning you to discuss it - what on earth is there to discuss?
My advice - forget it, and get on with your life. Choice stationery is a good company, and have deservedly won many awards. Phone them, they'll apologise profusely I'm sure.