Whilst onboard QE very recently, I paid the deposit for a cruise with my M&S credit card and had the booking passed to my agent as usual. The next time I went to Voyage Sales I was informed if I booked direct with Cunard and used the card it would attract 5% discount.
Cunard - Southampton said I had to oontact M&S for a code!!! M&S o not seem to know about it.