If you take sour cream and put it in a cheesecloth and let most of the water leak out, you essentially have cream cheese. Except that yours doesn't have starch, guar gum, or any of the other stabilizers/additives. So that's a great way to do it. Or you can use yogurt and do the same thing. Whatever you don't use, you can form into little balls and submerge in olive oil with some herbs.

Don't confuse FAT FREE sour cream with "Light" or "Lite" sour cream which is slightly less fat than whole sour cream and works fine for baking. It's ok for sauces too, but since in a sauce it is a flavor carrier, low fat sour cream makes a somewhat bland sauce. I use "lite" sour cream exclusively in dips, etc. Now, for baking, I often sub NON FAT (that's zero fat) yogurt when sour cream is called for. I have never had a problem. In fact, in baking I interchange buttermilk, lite sour cream and non fat plain yogurt. I realize there may be subtle differences but I have not noticed them much. Lower fat baked good have one issue though, they do not keep as long as the fuller fat ones. They dry out faster.

Several possibilities. Tofu can substitute. Get the soft stuff and whip it through a blender. If necessary, add a little cream to thin it down. Try draining the yoghurt. Line a collander with cheesecloth and pour the yoghurt into it. Let it drain until the thickness you like. Do not let it go overnight, or you'll have yoghurt cheese, and you'll have to thin it down again. Cream cheese thnnied with wither cream or the soured cream you have available. Let the cream cheese come to room temperature before whipping it with the cream. You can also thin the cream cheese with mayo if you like that sort of thing. BTW, don't they still sell clotted cream in Jolly Olde? Check that out as a possible substitute.

Linnea, the banana bread recipe I use calls for 1/2 cup of sour cream, so that's what I always use. Obviously, Greek yogurt has less liquid since the liquid (whey) is drained out of it. For baking purposes, your question regarding adding milk made sense. See if this (from Cook's Illustrated) might help when it comes to baking with yogurt... "We weren’t surprised with our findings when we baked batches of bran muffins and peach cobbler (with a biscuit topping) using these two different types of yogurt. In both instances, the samples made with Greek yogurt tasted slightly drier than those made with American yogurt. To come up with a formula to compensate for the difference in moisture content, we drained 1 cup each of plain whole-milk, low-fat, and fat-free yogurt in a fine-mesh strainer lined with cheesecloth; after two hours, 1/3 cup of liquid had been exuded from each sample. Armed with these results, we went back into the test kitchen, reducing the amount of Greek yogurt used in our recipes by one-third and making up the difference with water. The results? Tasters found the muffins and cobblers nearly identical. So to substitute Greek-style yogurt for American-style yogurt in baked goods, remember this rule: Use only two-thirds of the amount of Greek yogurt called for in the recipe and make up the difference with water."