This deal is available via the Broadband Choices website. It's £23 per month. After 3 months you are emailed a link to claim a £100 voucher. You can choose between a prepaid MasterCard, Tesco, Amazon or M&S. Factor in the £100 and this deal works out at only £17.44 per month.
Topcashback have £25.50 cashback for purchases on the broadband choices website. Hope it works as that would bring the price down even more!
Hope this helps someone.