A couple of years ago, Jonathan Naess, a City banker, was on his way back to the office after a lunchtime shopping trip when he was confronted by an unusual sight: a line of police officers announcing that they wanted to arrest him for committing a breach of the peace.

“I think I’d been chatting to someone in a shop in a way they found disconcerting,” recalls the 40-year-old father of one, talking in the foyer of a London hotel. “When we went to the station, they realised pretty quickly it was a mental health matter.

“It turned out I had been exhibiting many of the classic symptoms of manic depression. Distractability. Poor concentration. An element of grandiosity. I was sectioned under the Mental Health Act. I think the doctors and nurses in the locked ward were surprised to see someone arrive in a pinstriped suit.”

Listening to Naess discuss his experience is strange. Apart from the extraordinariness of what he went through, and that he is so softly spoken that it is impossible to imagine him raising his voice to make an emphasis, let alone commit a breach of the peace, it is unusual to hear someone so senior in the City – for more than 12 years Naess has been corporate finance lawyer at an international law firm, senior manager of regulation at the London Stock Exchange and a director and equity partner at Nabarro Wells, a corporate finance house – talk so frankly about an episode of mental ill-health.

It remains taboo in the City, though Naess, who returned to work after his treatment in hospital, is determined to change this. He has recently taken a sabbatical to set up Stand to Reason, a charity aimed at encouraging City professionals to talk about their experiences of mental illness in all its forms, ranging from stress, anxiety and depression to more serious conditions such as bipolar disorder and schizophrenia.

“When I got sick, it would have helped so much if I had known of a lawyer or banker or other professional who had had a similar experience and survived. With Stand to Reason, I want to bring together a range of professionals who have suffered from a mental illness and still managed to sustain a successful career. We want to do for mental illness what Stonewall did for gay people – show that it is not shameful, that it is quite possible to live a normal life with it, and even work.”

Naess, who gained a degree in Politics, Philosophy and Economics from Oxford, adds that he was fortunate to have an employer who had insight into mental illness, and was keen to help him to return to work, but that others are not so lucky. “They were keen to have an opinion from their own psychiatrist, but he wrote a positive and supportive letter, and I had supervision initially, which was as much to reassure me as reassure them, but I did two or three deals, and my own performance, measured in revenue to the firm, actually improved after hospitalisation. But for too many employers, the first thought is: how quickly can we get rid of the person?”

There is evidence for Naess’s assertion. A recent survey for the Shaw Trust suggested that British employers associate risk with employing the mentally ill in customer-facing roles – one in three think they are less reliable than other employees. Naess says he understands that it will take a long time to change such attitudes, not least because it took him years to accept his own illness.

He had his first significant episode and hospital treatment at 22, but refused to accept the diagnosis of manic depression or bipolar disorder, which affects one in 100 people and is typically marked by frantic highs followed by crushing lows. “When I found myself in hospital for the second time, it seemed too unfortunate an ‘accident’ to have happened to me twice. But it was only when I begged a nurse to be given an internet printout of the diagnostic manual, that I saw I ticked many of the boxes. It was an important moment. I finally realised I needed treatment.”

Naess now takes medication “as an insurance policy”, but understands why some sufferers with the same disorder, such as Stephen Fry, resist pills. “I think he said in his documentary that he quite likes some aspects of the manic stage, and is perhaps fearful of losing that spark. The problem for me is I stop being effective during that phase, so I’m very happy on meds.”

Naess starts to say that he has grand ambitions for his charity but suddenly stops himself. “I mentioned that one of the symptoms when I was ill was grandiosity,” he laughs. “But I’ll say it anyway. We want to totally change the way the mentally ill are viewed in the City. It’s the engine room of the UK economy, it is influential, and it is important place to change attitudes there.”

www.standtoreason.org.uk