The sweetest natural yoghurt that I have found is Sheeps Yoghurt. I think sheep's milk is slightly sweeter than cow's milk, therefore it does not taste so sour. The total sugar content listed on the information panel is 4% and I'm assuming that this is lactose.  It also states the following:

Calcium rich

No added cows milk

No added milk solids

No Sweeteners or Gelatine

And the ingredients: Pasteurised sheep milk, Live Yoghurt cultures

I got it at a Health food shop and I've seen it in David Jones.  Please note though it is more expensive. It's called Meredith dairy

Please, please tell me that this is OK!