Children have been treated at Great Ormond Street Hospital since 1852. For many with long-term conditions it is home. Amid the lights and bleeping machinery, families go through unimaginable heartache.

But where to find peace in built-up London? Up on the roof! The trees, plants and a pergola are transported from the Chelsea Flower Show by crane. As work gets going, Nick Knowles meets Sarah and Barry Day, whose two-year-old daughter Rosie is awaiting a heart transplant.

Emotions run high at GOSH. A place to sit, pass time, take the air, cry, grieve and relax is vital. Can Nick and co create a rooftop sanctuary?