- You may be looking for the Ghost Light actor of the same name.
Dr John B. Hallam was Melanie Bush's uncle. He was a historian and "dabbler" in chemistry who was a colleague of the Doctor's friend, Professor David Munro. His work centred on carbon dating, though Mel claimed much of his work was secret. He was responsible for Mel taking an early interest in science, which led to her earning a BSc (Hons) degree in computer science. Though fully entitled to use the honorific "Doctor" before his name, he tended to eschew the formality for the unadorned "John." (AUDIO: Catch-1782)
In 2003, John sent Mel an invitation to a centenary celebration for a scientific foundation in Berkshire, called the National Foundation for Scientific Research (UK). Mel and the Doctor consequently met up with Hallam and attended the event on 12 December 2003.
The Foundation was in fact partially situated on Hallam ancestral lands, and Hallam himself lived in a part of the largest house on its campus, as he still held the deed to it. He appeared to live alone in 2003, save for his pet cat, Jupiter.
In addition to his scientific investigations, Hallam was also the "Bush family historian", a hobby in which Mel was also interested. This knowledge of the past got him a "ticket" for a ride in the Doctor's TARDIS, when Mel was accidentally transported back in time to the Hallam home in 1782. The Doctor needed John's help, because John was the only available expert in the particulars of Hallam family history. In order to avoid changing Mel's personal past, the duo had to very carefully rescue Mel from John's house in the past, when it was the residence of their mutual ancestor, Henry Hallam. (AUDIO: Catch-1782)