Hi Marion, how long have you been making sour dough bread for? I started a sour dough starter about 10 years ago, but since my last baby a couple of years ago, I have to admit that I didn't have the time to cook, so I was tossing more starter down the sink than cooking. Have been thinking that I should start it again, as the kids used to love the chocolate cake that I made with it. Great to hear that your family also love the sour dough.
Ok back to yoghurt. With the runny one, I would guess that it probably wasn't cooked at a high enough temp, and the separated one too high and too long.
When using a culture I add a couple of tblsp of milk powder per ltr to give the yoghurt a bit more body, add the culture when the milk has cooled to under 45 c, then maintain temp at 42-43 c for 4-6 hours. I just put the milk into canning jars, and place them into a stock pot and cover with warm water. If I need to increase the temp I just turn on the stove the heat gentle until the temps come back into range. Then I let then cool at room temp for 1 hour, then put in the fridge.
When Mum used to make yoghurt from ready made yoghurt, she used to place the bowl of warm milk into a pre heated oven, which was then turned off, wrap the bowl in a thick warmed towel, and leave in the oven until morning.
Home made yoghurt always has a small separation of whey that will eventually cover the top of the yoghurt, that is quite normal. To make greek yoghurt, you can hang the yoghurt for a couple or hours to thicken it up.
Not sure if this has answered your questions, so please don't hesitate to ask more if I have missed something.
Hope your next attempt is successful. I know that it took me a couple a batches before I finally got it right.