"Napkin Niceties
The napkin should go on your lap once everyone has been seated. If it is a large napkin, fold it in half. If you leave the table briefly mid-meal, the napkin is placed on your chair. At the end of the meal, put your napkin to the left of your plate."

http://www.4hoteliers.com/4hots_fshw.php?mwi=494

But also, very wrong:
http://rjharrison.co.uk/index.php?option=content&task=view&id=43