An orchestrator is a person who interprets a composer's score to maximise its effect when played by a full orchestra. In this pursuit, they will be most concerned with determining which individual instruments play which specific parts of the music. Often, but not always, the orchestrator will also be the conductor of the orchestra.
In Doctor Who history, orchestrators were not formally credited until late in the RTD era, and then only on special occasion, as with The End of Time; during the Moffat era, the orchestrator began to be credited on a more regular basis. Until 2011, the only individual ever credited as an orchestrator had been Ben Foster, who also acted as the conductor of the BBC National Orchestra of Wales, Doctor Who's usual orchestra.
With the exception of The Doctor, the Widow and the Wardrobe, where Jeremy Holland-Smith provided "additional orchestrations", Foster continued to receive sole credit up to and including his work on series 9. He departed the role partway through the aforementioned series' production, with Alastair King taking over as sole orchestrator for the remainder of Murray Gold's tenure as composer. Beginning with series 11, following the advent of composer Segun Akinola, Alec Roberts took on the role.