At the time of writing this post there are still sizes 6,9,10 and 11 in stock but I don't suppose they'll hang around for long at these prices. Usually £12.99, they're now £4.99 (or £3.99 for size 6) with free delivery.
All I can tell you about Ben 10 is that he's a goodie who can turn himself into an alien and that he fights baddie aliens. What can I say. Little boys love this stuff. Demand for Ben-themed merchandise seems really high - I nabbed some Ben 10 PJ's and dressing gowns as Christmas pressies for some little relatives back in November because in my experience this stuff sells out fast. One week you spot it in the supermarket, the next it's gone. In fact I'm grabbing the next size up of slippers now, before I blink and miss them. Let's just hope Ben 10 is still the hero of the day by the time they're ready for a new pair of slippers.
Thanks to Holly100 on HDUK.