When Charles Middleton was 18, he presented himself at his local hospital and asked, somewhat naively, whether there was any voluntary work to be done.

Most medical avenues perforce being closed by his lack of any qualifications, the unimpressed authorities thought that his plans for a degree in mechanical engineering suggested a little light electrical work.

He found himself in a room full of bored elderly patients and a malfunctioning television set. “I just tapped the TV, as one does,” he says. “It sprang into life. So did the patients.”

It was an early lesson that the smallest of actions can make a difference to people's lives.

Mr Middleton, now 51, runs the UK arm of Triodos Bank, which provides savings products for ethical investors and lends money to or invests in socially and environmentally useful projects.

Being an ethical consumer today, as opposed to a die-hard environmental activist, means negotiating an array of micro-decisions - buying more expensive Fairtrade coffee, free-range meat or low-energy light bulbs, or taking the bicycle or bus rather than the car.

For investors, there are growing numbers of funds or products that are guaranteed to do no evil, by not investing in the arms trade or tobacco, or actually do good, by targeting worthwhile businesses.

These ethical choices have sprung up over the past decade and a half of affluence. The unanswered question is whether more difficult times will see many of those tiny decisions, which in aggregate add up to a socially conscious lifestyle, reversed.

Triodos will find out. Will its investment products, which provide a lower return than a typical ISA or investment fund, look less attractive as its customers' savings shrink? And will entrepreneurs who come to the bank for backing for social projects become more risk averse with their own funds?

In short, will the whole socially aware, green middle-class lifestyle turn out to be mere conscience-salving by the well-off?

Mr Middleton, predictably, thinks not. The bank's Dutch parent raised another €65 million (£51 million) from shareholders, more than twice the target amount, in the depths of the market meltdown last summer and savers show no signs of shunning its investment products.

He says: “While there may be some belt-tightening in evidence elsewhere, to date we've had one of the best years in our history in terms of people and organisations depositing money with us. There's no guarantee it will continue, but the numbers are well ahead of our expectations.

“It's tough out there,” he concedes, “but I think there are people who make conscious decisions about what they do with their life, and that includes what they do with their money. I don't think they think about it as a sacrifice - not in a pious sense.”

His earlier medical idealism notwithstanding, his background suggests little tendency towards philanthropy. He left Harrow as a largely apolitical teenager and spent three character-forming months on a cargo steamer in the Caribbean. Then, sidestepping the engineering degree, he joined the Army.

This was on a short-term commission sponsored by Barclays Bank, on the understanding that he would work there afterwards. His decision was influenced by his father, who had left the Army as a brigadier, and he ended up in his old regiment, 1st The Queen's Dragoon Guards.

“They give you a huge amount of responsibility very early,” he says. “It's very sobering.”

Inevitably it meant a tour in Northern Ireland, in an army outpost in the middle of a hostile West Belfast estate. “Things happened all the time,” he says blandly. “I really couldn't call it hairy. It challenged many of my assumptions about the way society worked.”

He came out of the Army and joined Barclays' graduate scheme, ending in investment banking in 1985, just ahead of the Big Bang and his bank's creation of Barclays de Zoete Wedd. Never a market trader, he took up the option of running the Bombay office, then applying for a banking licence.

He was introduced to the vibrancy of India, and the breathtaking poverty. “On my drive into work, people were thrusting limbs through the car window. You passed mothers with lifeless kids on their shoulders.”

He had taken up photography seriously in Belfast. There was no shortage of subjects in Bombay.

On his return to Britain, he mounted a couple of exhibitions of his work and sold the pictures to friends and business acquaintances. The £20,000 so raised put 17 deprived Indian children through an education.

He ran the Botswana office at a time when the Aids epidemic was rife and Barclays was campaigning for more openness on the disease. By 2003 Mr Middleton was back in the UK, “thinking I wanted to do something which was at the core more purposeful”.

He came across Triodos, which was created in the Netherlands in 1968, through a Dutch friend from Botswana, who invited him to attend the annual clients' day at the British units's Bristol base. Mr Middleton was captivated.

“It was one of those, what do you call them, Damascus moments,” he says. “I loved the sense that this was a business that was profitable but at its core had a purpose that I was excited by.”

With his banking experience, he was roped in to help in the search for a new head. “Halfway through the first meeting, I realised I couldn't stay in that group any longer because I wanted that job.”

Triodos has 20,000 savers in the UK. It claims to be the only bank in the country that publishes details of every loan or investment, 600 so far, in a glossy booklet.

On recycled paper, needless to say. They range from green energy projects to farming in the developing world. There are various community projects, including sheltered housing for the underprivileged.

Does Mr Middleton ever worry that his and his customers' micro-initiatives are dwarfed by the big picture, the headlong rush to industrialisation in India and China and all those coal-fired power stations coming on line week after week?

“If everyone says that, we will all go down the tubes,” he says. “You put your money in our savings account and that will make a difference, if only to one weaver in India, one person with learning difficulties in Birmingham.

“That's the challenge - get out of the sense that it's all too big, which we all feel at one point, and say we can do something, however little.”

CV

Born: Malaysia, April 1, 1957

Educated: Cheam School, Newbury; Harrow

1976-80: Army

1980-2002: Barclays Bank, including managing director, Barclays Bombay

2003-present: Chief executive, Triodos Bank (UK)

Family: Married, three children