The Last of Us is overwhelmingly critically acclaimed, and one of the most moving gaming experiences of the last ten years. No story spoilers here, please, but what really sells the game for me is the animation. Simply superb.
Left Behind is a piece of story DLC that is included in the Game of the Year Edition, or can be bought "Standalone". This is not like a preview or demo of the game - it really, really must be played after completing the main game.
I picked up Left Behind yesterday and just finished it a moment ago. If you just play it once, £3.29 is about £1/hour, I suppose, but I found it absolutely worth it.
Note that this is not a historical low, and you can find vanilla The Last of Us for US$10 for a US PSN account here and there, but £10 for TLoU and Left Behind together in the GotY Edition does seem to be the cheapest we've seen for a little while.