In most English blends it is the Orientals that provide the sour flavors, and the salty can come from the blends of different leaves. One of the reasons that you find so many different English varieties, even from the same companies, is because the amounts blended of each varietal leaf can change the overall flavors. A tiny bit of perique in a blend can be sweet and fruity, and a little more and the flavor gets spicy and peppery. Now, take in that the same differences in proportions of each leaf can greatly affect the overall flavor.
But, in my experience it is most likely whatever orientals are used that give the sour flavors. Some smoked straight can taste just like buttermilk.
