Hi Lesley
I too have problems with just eating a small amount of something and putting the rest back. My suggestion would be to buy the chocolate in the smallest packet possible. Cadburys do a Freddo bar which has 10.2g of carbs. They are normally located on the bottom shelf along with small packets of sweets aimed at children.
Chocolate peanuts are fairly low carb (for sweets) at between 35 and 40g of carbs per 100g, but usually come in 200g packets, the smallest packet I have found of them is in M&S at 125g.
Nutella, although fairly high in carbs per 100g, if you were spreading it some bread you had already allocated in your carb allowance would only add about another 10g carbs in a couple of teaspoons full. The risk with something like this would be to start eating it off a spoon from the jar...
I've tried the Sainsburys sugar free chocolate, it isn't very nice and the polyols they use in it can still raise your blood sugar.