Apmsam, I have made lactose free yogurt a few times with out any problems.(when the supermarket runs out I'd the milk I usually use). I now heat the milk for an hr at 90degrees, let it cool, add 6 drops of calcium chloride for 2 litres then use two different cultures from Green Living.Australia.
I always preheat my thermoserver then Wrap it in a fleecy blanket for about 12 hrs. No problems.