“Sadler’s Wells East will form the next generation of dance”Opening a mid-size dance venue has been on the mind of Alistair Spalding ever since he took charge of Sadler’s Wells in the early 2000s, and his dream is now becoming reality with the arrival of Sadler’s Wells East in London’s Olympic Park. He and fellow co-chief executive Britannia Morton take Siobhan Murphy on a tour of the building and reveal their ambitions for its future