First on the left hand side of the screen find a box called "information". Inside the box click redeem code. Type in shoptovoucher2 for £1 to be taken off. Sign Out for the code to work. Sign in. Find the 2100 points add to basket and simply checkout. Could not find anywhere cheaper for Microsoft points online.
Now that they have ran out of Stock visit play.com and add the Xbox live 2100 to your basket. Remember there Quidco and 5% off. Simply click the 5% off voucher to find out how to work it. http://www.hotukdeals.com/item/266921/5-off-absolutely-everything-play-co/