Hi Mal,
All of the Thorntons diabetic range carries a warning that it can have a laxative effect. That means that it is made with a sugar alcohol instead of plain sugar. That presents 2 problems to a diabetic
1) the laxative effect - from past experiences of others on this forum (myself included) I would say you were lucky
2) the sugar effect - sugar alcohols have 75% of the glucose content of sugar - so not much of a reduction.
This is why advice from Diabetes UK is that diabetics should NEVER, NEVER, NEVER eat products labelled as "diabetic", particularly chocolates. Because they are marketed as specially for diabetics, many diabetics believe they can eat them just like a non-diabetic would. They are almost as dangerous to a diabetic as ordinary chocolate.
The only chocolate that DUK endorses is the diabetic range made by Hotel Chocolat because these are made following advice given by DUK to the manufacturer not to use a sugar substitute but instead just use half the suger. Surprisingly they do not taste bitter - they are by a very long way the best I have ever tasted, but are pretty expensive and available only by mail order.