Huh -- it seems to say "sour cream" in the British version, too. (I got the recipe from Nigella's US Food Network show, but now I know it came from her book "Feast.")
I'm off to Sainsburys -- with a stop at Borders first to see what the cookbook actually says! Maybe I'll just buy both and see how many people have answered either way when I get back.

(I'm making this cake because you make it in a loaf pan -- makes it very portable, which is important since I'm traveling to this dinner party by bicycle! The recipe is called quadruple chocolate loaf cake, if you're interested.)
Kate