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.
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.
Szava Poirot when he talks about Colin's father calls him Superintendant, when Colin first visits him. So it should be Superintendant Battle.