Why do the Marchmains and their children have two different last names?
I've always wondered about this.
I remember two things very clearly: I am a great sinner and Christ is a great Savior.
Lord Marchmain is a Marquess (the Marquess of Marchmain), second highest rank in the peerage. As such he would have many other titles, one of them, perhaps, Earl of Brideshead. His eldest son would use one of those other titles as a courtesy title - Lord Brideshead. (It is only a courtesy though - the father holds all the titles.)
Flyte was the family name.
To take a real life example, Peregrine Andrew Morny Cavendish is the 12th Duke of Devonshire, the Marquess of Hartington and the Earl of Burlington. His son is known as the Earl of Burlington.