Well Mornington Crescent has been a part of I'm Sorry I Havent A Clue for about 30 years.
The original idea of the show, as it says in the introduction, is an antidote to panel games.
The comedy idea of Mornington Crescent is that is has no rules, they just list a load of london tube stations until the last person says Mornington Crescent (which is a tube train station in London)
They pretend that there are these long complex rules that have been changed and adapted over the decades, and some weeks they play "Johnsons rule 8, no diagonals" or some other such silly rule.
Each team member pretends this rule has confused them and debate its importance and how it will effect that weeks game.
I believe some years ago a joke book of Mornington Crecent history and rules was published.
Web sites
http://www.isihac.co.uk/
http://www.bbc.co.uk/comedy/clue/
http://en.wikipedia.org/wiki/Mornington_Cresce nt_(game)
http://www.amazonsystems.co.uk/data/morn.htm
http://www.mornington-crescent-rule.fsnet.co.u k/mornington-crescent-rules.htm