Instructions:Just heard this on the grapevine, and did it myself
Phone 0800 197 9722 Tell them your a swalec customer and you would like to claim your M&S voucher. They will ask you for your postcode and firstline of your address and your name and then tell you they will send the voucher.
A £10 if you just have electric with swalec, A £20 if you have Gas and Electric with SWALEC.
Happy Xmas All