JANE: Oh, I know about pretending. I once went on holiday and pretended to be twins. It was amazing fun. I invented this mad, glamorous sister and went around really annoying everybody. And d’you know, I could get away with anything when I was my crazy twin Jane.

SALLY: But you’re Jane.

JANE: Kinda stuck. It’s a long story.

Steven Moffat (b. 1961) Scottish television writer, producer

Coupling, 02×09 “The End of the Line” (2001-10-29)
    (Source)

(Source (Video) at 3:31; dialog verified)