Overview: Alan Titchmarsh, Carol Klein and James Wong take an exclusive peek at the Royal Horticultural Society's Chelsea Flower Show 2012 in this preview of the event. With show opening only hours away, they are joined by Nicki Chapman and Toby Buckland to see what gardening's greenest fingers are exhibiting this year in the Great Pavilion and show gardens. Joe Swift reveals the inspiration behind his first ever Chelsea garden, and Diarmuid Gavin unveils his high-rise pyramid garden.