should be to write a letter. Send it by special delivery, addressed to the sales director. Tell Scan that unless you receive the replacement within seven days you will immediately contact the Trading Standards office.
Unfortunately your course of action is somewhat limited in such circumstances, and although you will eventually get a result by using the legal process your best bet is to plug away with the phone calls. I don't have direct access to a senior person at Scan, but when you next phone the company you might try telling them that you've posted details of their inefficiency to tens of thousands of potential customers in the forum.