In 2007, Steve Jobs unveiled the first iPhone; Indie music ruled the airwaves; a relatively unknown Kardashian family made their TV debut and Brit IT girl Sophie Dahl was billed as the shrinking woman, dropping from a voluptuous 16 to a svelte size ten in just six weeks. David Marshall was revealed as Dahl’s trainer and soon Rachel Weiss, Rio Ferdinand, Gerard Butler and Kate Moss were all lining up to get lithe at his then Primrose Hill-based studio. The entertainment industry began to refer to him as the fourth emergency service.

Of course, a lot has changed since 2007, particularly in the world of health and fitness. London is booming with boxing, spinning, pilates and HIIT offerings, accompanied by Instagram-friendly backdrops. The £30 gym class, £7 protein shake and £110 round of cryotherapy have become normal elements of the modern fitness regime – but are such things really doing us any good?

“In my opinion, there is a letter missing at the front of HIIT,” remarks Marshall in his typically forthright manner. “People come to us after they have tried out their local gym or supposedly superstar trainer, and they don’t look back.”

Marshall is not merely a fitness trainer – he is the self-styled Bodydoctor. His enduring regime, which has scarcely altered in three decades, remains the only medically-approved programme available in London, supported by a little black book of the capital’s best practitioners. “We’re not a gym and we’re not a personal training centre. Our job is to make people healthy and well and the best that they can be.”

Located beneath St Peter’s Church on Eaton Square, Belgravia, Bodydoctor is a serene warren of fitness studios – all white walls and wooden floors, kitted out with state-of-the-art machinery. There are rarely more than two people training at the same time.

“It’s the antithesis of a KX or Equinox gym,” comments Tanya Rose, CEO and founder of Mason Rose PR agency, and Bodydoctor regular. “You go to train, not to be seen, and the programme is completely bespoke. I dropped two dress sizes within a month of signing up.” Rose’s 16-year old son also trained with Bodydoctor during the summer, transforming from “a string bean into a swimmer’s body.”

Bodydoctor alumni Lily Allen shed 19lb under Marshall’s guidance, dropping from a size 12 to a size 8. Allen then recommended Bodydoctor to her brother, Alfie, while he was working on Game of Thrones. He, Richard Madden and Kit Harrington soon had it written into their contracts that they would not remove their kit on camera without training with Marshall first.

As a teenager, Marshall was a keen footballer until a series of sports-related injuries put off him off the pitch. He became the guinea pig for his own technique, perfecting a sequence-based regime that eschews typical cardiovascular exercise in favour of long, lean muscles, coupled with a low sugar and low GI diet. The Bodydoctor book, which outlines his programme in relatable layman’s terms, is recognised across the world. “It’s about doing it properly. No quack cures, no starvation diets. We’re about making people well and giving them the information in a format they understand. You can do all kinds of stupid things for a quick fix, but it’s not sustainable or healthy.”

Above all however, a session at Bodydoctor proves that laughter is the best medicine. The real selling point at this temple of fitness is that David’s exercises are served with a healthy dose of outlandish humour and you will find yourself unexpectedly smiling as you sweat. Marshall regales wild tales and amusing anecdotes that distract you from the task in hand and his non-judgemental attitude and entertaining lingo are most likely why party-hard celebrities and time-poor CEOs feel at ease in his company.

“Behind the funny hyperbole and witty one-liners though, he’s not cavalier with his craft. It’s results and safety first,” comments hairdresser Nicky Clarke, who has known Marshall for more than 35 years. “He has a very different approach to anybody I had trained with previously.”

Most recently Bodydoctor has partnered with Marylebone’s prestigious John Bell & Croyden, pharmacist to Her Majesty the Queen, on an InResidence Fitness programme. An in-house nutritionist will create a bespoke diet plan that works in tandem with your training regime to improve your overall health and fitness. “The partnership has been designed for people looking to make positive changes,” comments general manager Alexander Johnston. “This could be post-natal, rehabilitative or even reversing the symptoms of type two diabetes – and this is why it is so unique.”

“John Bell & Croyden is an emporium of all things to do with health and wellbeing. When our clients need things, be it a supplement or a heart monitor or biometric scales, we send them there,” affirms Marshall. “They also work with everyone on Harley Street.”

So, what of my own experience with Bodydoctor? Already quite slim, I wasn’t looking to drop dress sizes in record time, but rather tone up ahead of an imminent wedding. Having spent a chaotic six months blindly navigating London’s vast fitness landscape, I felt jaded, overwhelmed and mentally low. I’d scrambled about lifting weights under dark lights; floundered on treadmills and nearly knocked somebody out in what was supposed to be a non-contact boxing class, while a sedentary desk job had left my muscles short and stiff. I’d thrown hundreds of pounds at something that had done very little other than dent my confidence.

At Bodydoctor, private sessions really were private (I was often the only one in the gym) and smoothly incorporated weights, cardio, pilates and yoga, along with a signature dash of jovial diversion. Within a few sessions, I not only exercised better but, more importantly, I felt better. Because ultimately, it’s not about fat or thin – it’s about positive physical and mental wellbeing. As Marshall himself puts it: “You only look good when you feel good; you only feel good when you are good and you’re only good when you’re healthy.”