If your office has a roof garden, chances are it’s a bit of green space with a few benches and the odd tree or shrub. But atop one central London office block – the Queen’s own bank, no less – there’s a secret garden that takes rooftop greenery to almost impossible new heights.

Coutts is the three-centuries-old banker to Britain’s royal family and aristocracy. That immediately conjures up images of mahogany panelling, Chesterfield sofas, thick carpets and walls groaning with Victorian portraits.