As described on this link:http://rewards.xbox.com/gamer-iq/
"Purchase Halo: Reach - Defiant Map Pack or Halo: Reach Games on Demand from April 1st to May 15th, 2011 and get 400 Microsoft Points."
(£3.33 for 400 points if you bought 2100 points for £17.50)
Thanks to shadowdogg for the original link! (http://www.hotukdeals.com/freebies/gamer-iq-challenge-xbox-live-rewards/911692)