Hi Ally,
I've just had a look at Asda online.
http://groceries.asda.com/asda-webstore/pages/landing/home.shtml#/product/910000284967
Their own brand Greek style yoghurt is made from milk, nothing added and has 6.6g carbs. (20p per 100g, 500g pot)
Their 'extra special authentic' Greek yoghurt has some cream in it and has 4 g carb (33.8p per 100g, 400g pot
Their own brand fat free Greek style yoghurt is made just from milk; probably skimmed, has 5.3g carb. (20p per 100g, 500g pot)
They sell Total fat free is made from skimmed milk, again nothing added has 4g carb. (47.6p per 100g, 500g pot)
The Total 2% fat yoghurt has 3.8g carbs (47.6p per 100g, 500g pot )
(much more expensive than the 'own brands'
As people have said these all contain just the natural 'milk sugars' that are part of the milk.
With normal portion sizes the difference in carbs is small.
Those that are full fat do have a lot more calories but if you are only eating the yoghurt with a few berries it won't amount to a lot of calories for breakfast and if it fills you up so you don't snack latter then it's better for you.