Not sure about mint ones. But I lived in Newcastle in the early 80's when Wispas were launched and we were a target reigon, so we were bombarded with free bars for several weeks. I was totaly addicted for a while too. Maybe it's something they put in the damned things.
Here's what happened. Cadbury's in their infinite wisdom, decided to streamline their product range. So out went Wispas and Caramels, in came "Dairy Milk Bubbly" and "Dairy Milk Caramel". Neither of which are quite the same as their predecessors. It'll be the death knell for Spiras next, I reckon.