WHEN Winston Churchill lives two doors down (perhaps he pops in occasionally to borrow a cigar or a magnum of Pol Roger) you know you are Belgravia upper crust. But that was taken for granted back in the day when the likes of Lord Louis Mountbatten frequented the royal residence that was No 37 Eccleston Square. Built in 1835 this stuccoed townhouse had once been home to a young Princess Victoria, while later Oscar Wilde used to picnic with his louche pals in the gorgeous private gardens opposite.