I'm not too sure that this answer is correct, but i've a feeling that its called "Return to Patagonia" by Lemon Jelly. The first album by Lemon Jelly featured a track called "Homage to Patagonia", and it sounds pretty similar to the one used on the Witch advert. Lemon Jelly's new album is out on the 21st October, and contains said track. Hope this isn't a wild goose chase, but I think i'm correct!
Actually, it's not Lemon Jelly at all, because I have just got the new album! Therefore, it is just a complete rip-off of Lemon Jellys "Homage to Patagonia" from the KY collection of Eps!