Taupin's lyric refers to the time in 1969, before John was a popular musician, when Elton was considering marriage to his girlfriend, Linda Woodrow. Elton and Linda were sharing a flat with Bernie on Furlong Road in London’s East End, hence the opening line “When I think of those East End lights.” While having serious doubts about the looming marriage, John contemplated suicide. He took refuge in his friends, especially Long John Baldry, who convinced Elton to abandon his plans to marry in order to salvage and maintain his musical career. As a sign of his respect and gratitude for Baldry, Taupin wrote him into the song as the "someone" in the title.
From Wikipedia:
Taupin's lyric refers to the time in 1969, before John was a popular musician, when Elton was considering marriage to his girlfriend, Linda Woodrow. Elton and Linda were sharing a flat with Bernie on Furlong Road in London’s East End, hence the opening line “When I think of those East End lights.” While having serious doubts about the looming marriage, John contemplated suicide. He took refuge in his friends, especially Long John Baldry, who convinced Elton to abandon his plans to marry in order to salvage and maintain his musical career. As a sign of his respect and gratitude for Baldry, Taupin wrote him into the song as the "someone" in the title.