Being a peer isn't what it used to be. Now that their Oxfordshire estate has fallen on hard times, Lord and Lady Gilroy have had to fall back on a series of increasingly desperate money-making schemes. Their latest, an Agatha Christie weekend with eight guests and a fictional murder, nearly founders before it's launched when the Christie estate forbids the use of any of the old inventer's plots or characters. Luckily, none of the guests--decorous stock characters, all of them, though duly equipped with the modern complement of nipples and thighs--is terribly disturbed by Lady Deirdre's last-minute substitutions, since they all have their own fish to fry: an adulterous affair, an article for a true-crime magazine, an attempt to strong-arm Lord Algernon into parceling out the estate for a subdivision of homes. For a while, Lady Deirdre and her first-time author have fun confusing fictional threats with real ones, but once the two murders, fake and real, are discovered, everything that follows- -despite the local inspector's sage observation that ``few things about the Wittenham Park murder seemed to be normal''--is utterly predictable. A miasmal lack of surprise will dampen the weekend for all but the staunchest nostalgia buffs. If only the Christie estate had relented and allowed a dollop of Agatha's surpassing ingenuity.