Why is my Frozen Yogurt Grainy? I used Cal Peternell's recipe:

1 cup of whole milk
1/2 cup of half and half
1/2 cup of sugar
pinch of salt
3 cups of whole milk yogurt
zest and juice of a half lemon

BUT... Instead of half and half I replaced it for cream because this is what I had at the moment. Could that be the culprit?
Quicker freezing = smaller ice crystals.  One hour is a long time, that likely contributed to your grainy texture.  If the Breville ice maker is your only option, make sure your base is as cold as possible before using the ice cream maker.  That means refrigerating it for several hours in advance.  You also might want to strain your base through a fine sieve first.  This would help to ensure that the graininess is not coming from your ingredients.