Gwen Moffat - bohemian, author and Britain's first female mountain guide - was the subject of the multi-award winning film Operation Moffat, released in 2015 by Light Shed Pictures. Now aged 93 and enjoying some quiet time after the hubbub of Operation Moffat's success, Gwen watched Psycho Vertical - Light Shed Pictures' latest biopic based on Andy Kirkpatrick's autobiography of the same name - and felt moved to write about her impressions of the film...

I came to Yosemite late on a summer's afternoon, the sun behind me, the road dark and empty in the trees, and, as I drifted round a bend the world bloomed suddenly, brilliantly. Realistically it is no more than a big wall but on that first view it was thousands of feet of sun-drenched granite reared above the foliage and it blocked the sky: El Capitan.

I came, I saw, I hung around. Ostensibly researching pioneers' routes I explored the high country: the domes, the soaring ridges, the crazy rims of the Valley where eagles floated across the void. One morning early, sleeping high, I woke with El Capitan still night-cold and black and the sun came up striking the east face, and light ran down the vertical edge and that almost imperceptible flare at the base: three thousand feet of laser beam.

How it was climbed, where it was climbed, didn't concern me. For one whose longest rock route was maybe on Skye with a hundred feet of rope and a couple of carabiners El Cap was not just out of my league, it was out of my world. It was Olympus, the haunt of gods. And then friends climbed it and it was humanised; I found a connection however nebulous. There was a glimmer of comprehension: pitches, stances, companionship, even sleeping on the wall...I had been benighted, spending the small hours praying for the dawn, trying to keep my partner awake – but to climb El Cap solo, without companionship: alone on that wall? An investigative reporter, I didn't ask questions. I speculated but imagination failed, I didn't want to know.

And then I watched Psycho Vertical and I was blown away. For an extended period I couldn't believe that Kirkpatrick was solo; vaguely I accepted that this was a dummy run, a recce with an unseen companion. Slowly it was borne in on me that this was the real thing, that he was indeed alone. For all that anyone else showed on the long shots here was a man forging his way up a gargantuan tower with the aid of a load of stuff weighing as much as himself. This: food, water, bedding, a millstone of metal and ropes, he must haul up to every perch, real or manufactured, a performance that obviously involved more expenditure of energy than the actual climbing. All the time, in the background, always in looming focus was the rock, every block and overhang on a larger scale than rock should ever be and yet, in inverse proportion and with terrible significance, the holds didn't show.

Pictures are seared on the brain: Andy, his feet dangling, sitting on the outer edge of his bed that was no more than a strip of plastic suspended on lines above nothing. And, higher, there are wide sweeping swings: a spider on a filament, back and forth across the face as he attempts to lasso a spike which anyone can see is so precariously balanced that it will come off at a touch. Indeed there's an expert watcher on a view point insisting in voice-over that Andy's doing it all wrong, but he does it his way and the rope drops over the flake like a charm and he pulls up and everything stays firm and the watchers cheer.

The film is based on the autobiography. Kirkpatrick Senior was in RAF Mountain Rescue and Andy's early days were good in Merioneth, less so when the family moved to Hull. For a while his story comes over in episodes, unmemorable: schooldays and the difficulties with reading followed by dead-end jobs; the first climbs and walks that lead to the Alps, to the Arctic, Antarctica, all the lonely wildernesses, but other adventures, even the fraught alpine routes, glossed over: unremarkable interludes behind the running story where the camera lingers lovingly even on the preparations: on the man sorting his gear, interminably counting and sharpening, preoccupied and isolated in the wooded camp site under the great tower. He talks continuously: about Life, his own life: self-deprecating, self-analysing, an articulate bear.

None of this would exist without the camera. Jen Randall was the director and her own cameraman, with help from Ben Pritchard on the wall. Those amazing close-ups of gnarled fingers reaching to place yet another piece of metal in a crack, one more micro-chip of security, that shot of dangling feet above the abyss, the pendulum swings through space – where was Randall then? On a parallel route? Hanging on a sky hook? On Andy's shoulder?

She was responsible for the editing; the stills, old film clips, archive material from Wales to Patagonia; jigsaw pieces tweaked and teased into a coherent whole. There are flaws of course, the wrong snatch of music here, too raucous there, but all eclipsed, epitomised by one memory: of pearled granite, a horizontal crack in a vertical world, and long moments of silence broken only by the tapping of a hammer in the great stillness. Magical.