Sir David Attenborough: "There are many virtuous people who can't walk at my age – it's a matter of luck"

The conversation was the culmination of a year's filming, embarked upon to promote an ambitious project the Queen has been championing to create a protected canopy of trees second only in size to the Amazon rainforest, persuading Commonwealth leaders from more than 40 countries to sign up. It's the first major environmental initiative to which she's given her name.

"I'd observed the Queen on several occasions across the year handing out Queen's Commonwealth Canopy certificates to Commonwealth heads, and had admired her unflustered and inquisitive manner, often broken with a broad smile," says Treays. "But I'd never seen her so relaxed and delighted at being outdoors with someone her own age, similarly revered and arguably just as famous."

Treays describes the pair's "complimentary sense of energy, adventure and fun", and writes "on occasion it was hard to keep up."

For Attenborough, the Queen is "very unsolemn, very good at putting people at their ease", but their shared mobility is not something the broadcaster takes for granted.

"We must be very lucky in our constitution," he says. "There are very many virtuous people I can think of who can't walk at my age, so it's a matter of luck isn't it?"

Read the full features on The Queen's Green Planet in this week's Radio Times – on sale in shops and on the newsstand from Tuesday 10th April