KEEP IT PERSONAL

I was just wondering, when sending out query letters to try and secure representation, shall I address them to a certain person within the company (any agent listed), or merely the address listed in the HRD?

To an individual person if at all possible. It's probably a good idea to ask the receptionist, nicely, what junior agent might be open to a new client. But if you go to the top first, sometimes they'll send you to the correct underling. Anyway, always to a person.