Are you sitting comfortably? Then I'll begin.
originally an 'ad lib' in 1950 on BBC Home Service radio programme Listen with Mother. Used as the consistent opening line until end of series in 1982.
BBC web-site - Accessed 25 Jan 2011
Partridge, Eric (1986). A Dictionary of Catch Phrases:British and American, from the sixteenth century to the present day. Routledge. p. 26. ISBN 041505916X. Note that Frieda Fordham (a psychologist who advised the BBC) has also been credited with it.