I always use woodlands dairy sheeps yoghurt as a starter base for my homemade yoghurt, but I think my parents used a yoghurt starter from English suppliers, I will ask them and get back to you, but the yoghurt has always been very smooth. Another thing which is quite good is instead of cooling the milk in cold water, I sometimes leave it to cool for 10 mins in the pan on a wooden board and that has worked in the past, but the texture with doing that can fluctuate. What temp do you add the yoghurt and how long do you keep it on for?