Elena I don't remember the book saying what his last name was, but maybe I missed it. On Wikipedia, it said that Colin was Super Intendant Battle's son, but I'm not sure if that's true or not. Sorry I can't be of more help.
Szava Poirot when he talks about Colin's father calls him Superintendant, when Colin first visits him. So it should be Superintendant Battle.
Greg Stephen, I didn't know, but did realize somewhere in the past there was a name. (Responses below.) And I LOVED the reference to Chandler's "Big Sleep" adult-borrowing-bookstore. In "Clocks", Lamb finds his way to his bosses offices through a messy book store, and Christie mentions a 'curtained area' off to the side and unnoticeable except for those people looking for a certain kind of material. In "Big Sleep", a villain actually runs a 'porn-borrowing-bookstore" (but in the 1930s, this may have been stocked with "Ulysses" and medical-type books."). The reference is so obvious because Christie talks much about American crime authors.