Hi Michael,
Well done on sticking with it, and great to hear that it seems to be working for you!
The best dark chocolate from the point of view of low-carb is the 85% stuff; Lindt and Green & Blacks both do them, and I think Sainsbury's do their own-brand version as well. From memory it's something like 19g carbs per 100g, as opposed to the 36g in the one you just posted; so roughly half the carbs. It took me a bit of getting used to though, but I love it now (not that I eat it often, maybe twice a month; and find that about 20g at a time gives me a fix, rather than scoffing an entire bar of milk chocolate!)
The chocolate you've posted uses polyols in place of sugar. These are supposed to have a smaller impact on blood glucose than plain sugar and they contain slightly fewer calories (so from that point of view polyols "should" be better for the diabetic), but in reality many people find that there's not much difference. Personally, given the choice, I'd simply go for the 85% stuff that's made with real sugar, and not eat too much of it.