Dont know why it hasn't been posted yet but I just bought 3 x boxes of 10 x 440ml tins of Draught Guinness for £20 instore.
Roughly 67p per can!!
Cheapest I could find! ;-)
Also available online but only available for deliveries on or before 20 December 2009. To take advantage of this offer, add 2 items (or multiples of 2) to your basket.