Here is a very rare recording of Amy Winehouse singing I Saw Mommy Kissing Santa Claus. I think it's from the Q Awards in 2004. Side note: The Pet Shop Boys were award the Q Inspiration Award and The Red Hot Chili Peppers won the Best Act in the World Today at the 2004 Q Awards.
Anyway, I'm a huge Amy Winehouse fan and and love this version of the song. The only place I've seen it released is on a Dutch produced CD compilation called Best of Christmas.
You can buy it at Amazon.