"Louis Serbe" is the name by which composer Louis Febre (or Febré) was credited on the 1996 television movie. There, he acted as a sort of assistant to mentor John Debney. He was formally credited with "additional music".
The two would enjoy a partnership thereafter. Almost immediately after Doctor Who, the two jointly won an Emmy Award for their work on the score of The Cape. His work tends have been for other television movies, but he gravitated towards two Warner Brothers properties in the 2000s. He became something of a house composer for the Scooby-Doo movies of the early 2000s, while in 2007 he became the main composer on Smallville, finishing out that show's last four years. He's also scored some theatrical movies in the 21st century, including Swimfan.
It's unclear why his name is spelt "Serbe" on the Doctor Who credits, as he's overwhelmingly credited "Febre" on other works.