Wonderful morning. The first of the lakes of mist in the valley. Dew on cobwebs. Rams curled in hollows in the early sun. The garden giving forth potatoes, carrots and soon peas.”

If you find it hard to bear the chasm between this idyllic rural dawn and your actual mornings spent sweating on the Tube en route to your desk job, you are not alone.

British city-dwellers have been swapping their Oyster cards for country cottages and Range Rovers in increasing numbers: in 1983, 190,000 people left London for the countryside; by 2003 this had increased by 38 per cent, to 262,000. Since 1981 English cities have lost 2.25 million people to the country. Countless more have weekend homes in rural areas. Television is awash with programmes for the armchair farmer, such as Build a New Life in the Country and Jimmy’s Farm.

Yet this idolising of the country can be naive. The reality is not always so charming. Horatio Clare’s bittersweet memoir of a country childhood, Running for the Hills, describes the drawbacks in painful detail. Horatio’s parents, Jenny (from whose diary the opening quote is taken) and Robert were middle-class London media types who decided in 1970 to leave their comfortable city lives and pursue the down-sizing dream.

They bought a remote, picturesque Welsh hill farm where they planned to marry, have children and live happily ever after, far from the dangers of the soul-sapping city. The farm was a dump. Dilapidated and at the top of an incredibly steep dirt track that was impassable in winter, the house was draughty, rotting and not so much damp as soaking, with no bathroom, just a chemical lavatory in a shed. It came with 72 acres and 100 sheep. Jenny and Robert knew nothing about sheep.

The book describes the horrors of learning sheep farming from scratch — manually scraping handfuls of maggots out of infected flesh and decapitating dead lambs mid-birth. The farm ran at a hopeless loss. The birth of Horatio and his brother Alexander coincided with the first cracks in Jenny and Robert’s marriage. Eventually, Robert decamped to London and a job at the BBC, from where he sent cheques. Jenny stayed, bearing poverty, loneliness and back-breaking work. She moved down from the mountain in 1989 (with her sheep) only when a love affair ended painfully and the children left for boarding school.

Horatio, now 32, a BBC researcher-turned-writer, lives happily on a houseboat in Marylebone. He recalls: “It could be fearsome, the f**king weather, and no comfort anywhere. Dirty and depressing and mud everywhere. Mum would shout, ‘These bloody wellies, I used to have such nice shoes!’ ”

Even as a young child, Horatio was aware of their poverty. “Alexander and I used to fantasise about a world made of duvets, carpets, sofas and cushions, where we would eat white bread. We had none of the comfort food that the rest of the world seemed to eat.”

They also lived in fear of crime. The mugger paranoia of city-dwellers had nothing on the Clares. Horatio remembers “the very worst bit” as being “the fear of not being safe at night”. “All the years we were up there [Mum] was never more than half asleep; she was constantly on guard.” In contrast, London, on his visits to his father, “felt completely safe. It just seemed to be a controllable world”.

The children’s education also suffered. “Even at the time, we knew we were missing out. It allowed us to cruise, so we did. We would definitely have done better in London.” Weekends and holidays were spent helping their mother with the farm. “She used us ruthlessly! As soon as we could run we were sent to head the bloody sheep off.”

There were, of course, wonderful moments for young children. “As soon as you opened the front door you were in Paradise. We were completely free.” Indeed, much of the book is a love-song to nature.

Being a teenager up a mountain, however, was “hell”, says Horatio. “If friends, music and the opposite sex are the teenage holy trinity, then the country is a kind of purgatory of woods, fields and hills, when all you want is kicks.” Starved of music, television and fashion, he says that “for the rest of my life half my generation’s cultural references were a mystery to me.”

And there were other consequences. Sent to Malvern public school at 11, Horatio wholeheartedly embraced the pleasures of society and was expelled for smoking cannabis — a habit he continued to indulge heavily at the University of York. As a reaction against his upbringing? “If I’d lived in London maybe I’d have grown up faster. I had a bad reaction to my childhood in my twenties — I was on the whole suspicious of authority. I’m not blaming my mother for my drugs habit, I blame myself, but my indulgence of it was something I culled from my upbringing.” Alexander, has, by contrast, the “pink, clean lungs” that Jenny wanted so badly for her children.

Horatio believes his mother would never have stayed on the farm after the separation had she not had children. “She was desperately lonely at times. She made a huge sacrifice for us. The alternative would have been to carry on in journalism in London, but her line was, ‘I don’t want to be the only person at a bloody dinner party who hasn’t got a husband, with all the women looking at me like I’m a threat’.”

Horatio is keen that townies planning a drastic downsize realise that living in the country is “not a pastoral idyll”. His advice is to do it only if you are prepared to “put your whole heart into it, because “you’ll go bloody mad, just desperate, and London seems so exciting and so beautiful.

“Even in a London garden you see the horror of maggots eating a baby bird; you shouldn’t make the mistake of thinking the countryside is all butterflies and beauty.”

Some names have been changed.

Running for the Hills by Horatio Clare (John Murray) is priced £14.99.