I've never had the Potato Leek Soup at Beirgarten but I have had it at the Rose and Crown and it is amazing!!
I'm not sure if it's the same but the recipe for Potato Leek Soup at Rose and Crown is:
Recipes - Potato Leek Soup - Rose & Crown - Great Britain - Epcot
I llllllllllloooveeee this soup and made it at home once and it was good. Not the same as being at Disney though.