Summer of Arcade may be over, but the great deals on Marketplace just keep coming. During the month of October, spend 2400 Microsoft Points on Arcade games and/or game add-ons and you’ll get 800 Microsoft Points added to your membership as a thank-you gift. Spend your bonus points on anything you want—game add-ons, avatar accessories, or even another Arcade game!
Put £17 down as the cost presuming you manage to pick up a 2100 point card for £15 somewhere. Meh, you do the maths :p
Small print says it may take Microsoft until the end of November to send you your points.