I poured 8 cups of whole milk into the crockpot, set to Low. The milk was slowly heated to 180F/82C (about 3 hours) then I unplugged the crockpot and let it sit until the temperature dropped to 115F/46C (about 3 hours). At that point I added 1/2 cup of the yoghurt I made last week (as the starter), wrapped the crockpot in a thick towel and let it rest for 8 hours (well, until I got up this morning.)
It thickened a lot better than last time - keeping an eye on the temperature definitely worked! So now I have 8 cups of beautiful plain yoghurt to enjoy for breakfast this week. So simple and so nice.