A garden square in one of London's most exclusive postcodes has become an unlikely symbol of prime residential property's malaise. Several mansions worth upwards of £15m have languished on the market for two years or more, with vendors repeatedly slashing asking prices only to find buyers still unwilling to commit. This is not an isolated curiosity but a warning light for the entire prime central London (PCL) market, where transaction volumes have fallen by an estimated 15-20% year-on-year and average discounts to asking price now regularly exceed 10%, according to agents tracking the £5m-plus bracket.