A straight-backed, well-spoken former management consultant and ex-soldier in a wax jacket might not resemble much of a tree wizard, but the man leading me into a steep Cornish valley of gnarled, mossy oaks is called Merlin. He possesses hidden depths. And surfaces. Within minutes of meeting, as we head towards the Mother Tree – a venerable oak of special significance – Merlin Hanbury-Tenison reveals that he recently had a tattoo of the tree etched on his skin. I’m expecting him to roll up a sleeve to reveal a mini-tree outline, but he whips out his phone and shows me a picture: the 39-year-old’s entire back is covered with a spectacular full-colour painting of the oak. “It took 22 hours. I was quite sore,” he says, a little ruefully. “But I was in London afterwards, feeling quite overcome by the city and I had this moment: I’ve got the rainforest with me. Wherever I go, I feel like I’m carrying the forest and its story with me.”

Merlin is keen to tell the remarkable 5,000-year story of this fragment of Atlantic temperate rainforest – a rare habitat found in wet and mild westerly coastal regions and which is under more threat than tropical rainforests. In fact, he is now the custodian of this special, nature-rich landscape filled with ferns, mosses, lichens and fungi. He is slightly more reticent about his own remarkable life. Both stories are well worth telling.

Cabilla, a 250-acre hill-farm on the edge of Bodmin Moor, was bought in 1960 by his dad, the explorer Robin Hanbury-Tenison. He wanted a place where he wouldn’t hear traffic and could drink from the stream. The call of a song thrush, not traffic noise, fills the valley to this day and Robin, now approaching 89, still drinks from the stream. Merlin has taken over the farm and has conjured up three big visions: he wants to expand the less-than-1% fragment of Atlantic rainforest that endures in western Britain; he hopes to demonstrate that a new kind of hill-farming is viable and employs local people; he also seeks to open up such forests to those who need them the most – the traumatised, the broken and stressed urbanites who don’t even realise they can thrive if they take time beneath an ancient tree, imbibe the smell of damp leaves and listen to the river dancing over the ferny rocks below.