It seems we came to England at the right time. On the way back from Aldi, I dropped in on Thornton's in Spring Gardens (if you know the geography of Buxton, that's like going from stage left to stage right via the lighting box). It's a chocolatier that I remember from our last two trips as being good, but a tad on the expensive side.
Turns out that it's their hundredth year in business, and they're celebrating with some decent discounts! I picked up four 230g boxes for 10 quid, and a couple of "lucky dip bags" of unknown content (but guaranteed to be worth at least 18 pounds) for the same price. Stocks are nearly out, but if anyone else has a Thornton's near them, it'd be a good time to drop in and see what they have!