HASINA Begum is screaming.

Her neighbours are shouting at her in the narrow concrete corridor outside her one-room home in the slums, and she is screaming back at them as she works her thumbs to free peas from yellowing pods on the ground.

Bundles of filthy string hang from bars on her only window. Sacks of clothes are piled high in the corner. Outside, in the alley below, untreated sewage and anonymous grey liquids seep through the stones. Incense sticks dry in the sun. Handmade benches are varnished in the street next to stalls crammed with pineapple and watermelon. Cows wander past waste dumps, barefoot children, colourful rickshaws and rusting bicycles. There is nothing in the surrounding slums to hint at Bangalore's reputation as India's Silicon Valley.

This city of 10.3 million people is home to many of the world's leading hi-tech industries – Dell, Microsoft, Yahoo, Infosys, Capgemini. Every fourth employee of IT giants IBM, Oracle and Accenture is said to be in India and most are here. Bangalore now has more pubs than any other Indian city, some five million vehicles, dozens of designer shopping centres and top hotels. Ornate Hindu temples stand next to new ring-roads and flyovers, roadworks for the metro system. The disparity between the poverty and the new sliding glass doors is striking. Stigma, poor education and superstition mean the disabled reap little benefit.

Earlier this month it emerged that the French had won a contract to produce Indian fighter jets worth £13 billion. Apoplexy ensued because Britain gives £280 million a year in aid to India and yet was pipped to the post in this deal. A number of MPs called for an end to UK aid to India. But in the 1980s it was agreed that aid should not be based on trade – particularly not arms deals, and that it would be aimed at people not countries.

Hasina, 35, is one of tens of thousands still living in the city's slums. Illiterate and vulnerable, she was sold and sexually exploited as a teenager. Her husband earns 300 rupees (about £4.20) a week when he can find labouring work. Her son is 10 but she says he refuses to go to school. Some days he earns 20 rupees (about 28 pence) a day. They struggle to afford two meals a day.

"The doctor has refused to give her a medical certificate to allow her to draw a mental health pension because physically she's not disabled," says Selvi Armugam, her community social worker from the Association of People with Disabilities (APD), a Bangalore-based charity part-funded by the Scottish Catholic International Aid Fund (SCIAF). "She's been diagnosed with schizophrenia but it's always very difficult to get the mental health pension of 400 rupees (about £6) a month.

"She's from Andhra – a very poor rural state – and the Muslims there are the poorest of the poor.

"Her son threatened to kill himself if she forced him to go back to school. He poured kerosene on himself so she ate all the matchsticks in the house. She's still eating matchsticks in case he tries again."

She sits in squalor, tormented by the voices that haunt her mind, conscious of some level of injustice but unaware that her home city is becoming so prosperous.

"People cheat me," says Hasina. "A man sold me a ration book for oil and rice. The food didn't come. I went to the police but they chased me away. When the police have taken a bribe they don't help.

"I worked making incense sticks but they didn't pay me. People think I'm mad so they take advantage. There are many criminals here because it's a slum."

She says she is disturbed by what happened to her as a teenager.

"In my village I worked in my uncle's house," she says. "He worked me so hard I fell sick and so he arranged a marriage for me. I was 14. My husband was mean. He already had three wives. He took me to Kuwait and sold me to some men who beat me and sexually assaulted me. After that I was sold to many people. Then I was brought back to Bangalore."

Hasina had hoped that her son would be a way out of her situation; that he would become well educated and get a good job. But she says he refuses to go to school. He is illiterate.

ENVELOPED in the stigma of his mother's mental illness, he is as trapped in the slums as she is. In 1984, 17 out of 20 people in India lived on the equivalent of less than $2 a day and more than 50% lived on less than $1.25 a day – the international threshold for poverty. By 2004, there were still 300 million Indians living in dire poverty.

India has experienced rapid growth since 1991, which still stands at about 8%, but there has been little significant reduction in poverty or hunger.

"In Bangalore, the prosperity is very much linked to IT and the service sector," says Chiranjib Sen, professor of economics at Azim Premji University in Bangalore.

"These IT jobs are very well paid but there are few of them and the IT sector cannot integrate huge numbers of people. It is a magnet of growth but can be a great spreader of inequality. In many ways Bangalore is a make-believe modern city," Sen explains.

Not far from the city's new immaculate, glass-fronted apartment blocks and designer shopping centres, Sagai Kalyani, 35, spends her days sitting on her bed in darkness in Djalli slum. She usually leaves the room only on Sundays when she attends church to pray for a cure.

She starts crying when she mentions her family. "Sometimes there is not enough to eat," she says. "My brother earns about 200 rupees a day (about £2.80) when he can get work. Some days he doesn't find work. I still believe God could cure me in the future. I read my bible every day."

Sagai was 15 when she got sick. The family say she stopped eating and talking.

"Ten years ago I was taken to all the shrines and worship places," she says. "They believed I had been possessed by a devil and so I was beaten with a broomstick every day for a week to drive out the spirit.

"I thought I needed the treatment to drive it out but it made it worse. I used to hear voices. On my wedding day I went to my husband's house but I didn't like it. So I ran away."

By the time she was identified in the community by APD she was very distressed. "She was aggressive," says APD worker Selvi Armugam. "She used to take off her clothes. For three to four years she had been sitting on this bed alone in the darkness. She had little food and never went out."

Sagai is from the Dalit caste. Dalits, formerly known as "untouchables", are at the bottom of the Hindu caste system in India. They still face widespread suffering and segregation. Sagai faces a double level of discrimination. It means her brother can only do menial work.

"Bangalore is one of India's most caste-ridden cities," says Prabhakaran Rajendra, an independent consultant and caste expert. "The slums are a reflection of the caste system. Caste is a social and political disability."

Although caste discrimination is illegal, biases remain in many areas.

"Some 70-75% of people with a disability are from the lower castes," says Krishnapura Gopinath, programme director at APD. "The real causes may be malnutrition, poverty, poor healthcare and pregnancy care.

"The mindset of people – whether literate or illiterate – is that mental disabilities are some kind of evil curse that has got into the body. Some still think that of physical disabilities, too. There are many superstitious beliefs and so few psychiatrists."

There are an estimated 70 million disabled people in India. Growing demand for employees means the private and public sector cannot ignore them all.

"In 2011, for the first time the census asked if there was a disabled person in the family," says Sebi Chacko, convener of the Disability Forum of the Confederation of Indian Industry and head of HR at Thomson Reuters for South Asia. "In this war on talent, the corporates are missing out because they are not employing the disabled. That is something we're working to change. The government is setting a mandatory quota of 3-5% of disabled in the public sector.

"India really opened up to the world in the early 1990s. Since then there has been unbridled growth but is it fully inclusive growth? The answer to that is no. What we see is the growth of the middle classes."

Glory was 13 when a friend of her father's said a "cure" had been discovered for polio. For the next three years she and her mother spent most of their time in the village of Vellore, 300km away.

"Every day I had to take a cup of rum with pigeon's blood mixed with pachaela leaf," says Glory. "It was expensive but my father wanted me to be better. The whole family suffered.

"We went there for three years but then I got really sick. I spent six months bleeding. Doctors said I was going to die. I refused to go back."

There are some disabled people who break the mould. Glory now teaches children with polio, cerebral palsy, rickets, the deaf and the blind. Created in a garage 52 years ago by a small group of people with disabilities, APD has helped 23,000 disabled people. It offers outreach work to slums and surrounding rural communities, lobbies the government on rights and legislation, offers schooling to more than 200 children, provides physiotherapy to more than 6000 people a year and helps with mobility aids and mental health care. It is run with the slick efficiency of the nearby multinationals.

"I have a salary which makes me independent and means I can help other people," she says. "I have hands, eyes, ears, nose, mouth. I am a normal girl. Let us work, let us be equal."

Amid the lowing cows, the blaring Bhangra, the ferocious honking of horns, it is a clarion cry for equality of opportunity between the most disadvantaged and the millionaire businessmen in India's Silicon Valley. While the arguments continue about why a country with such a fast-growing economy would receive aid from Britain, Glory continues to teach and APD continues to transform people's lives.

"No-one wants to be a burden," she says. "With APD everything is about work and independence."

Some of the further education and IT courses run by APD, and the workshop where the disabled make wheelchairs and prosthetic limbs, are becoming self-financing, but for the most part it is not and now the people it helps are in desperate need of help. In 15 or 20 years' time, APD, like India as a whole, may no longer need that help. But for now it does.

HOW YOU CAN HELP

The Scottish Catholic International Aid Fund (SCIAF) has been working with vulnerable people in India since 1985. Operating through local partners including the Association Of People With Disability, SCIAF provides mobility aids, prosthetics, physiotherapy. education and employment for disabled people, as well as seeds, tools, training and micro-finance loans to poor rural families.

Having begun in a small classroom in Rutherglen in 1965 with a budget of £8000, SCIAF now spends more than £5.5 million providing long-term development support and emergency aid in Africa, Asia and Latin America to hundreds of thousands of people affected by conflict, hunger, poverty, injustice and disease, regardless of their religion.

The Wee Box, Big Change Lent campaign runs from February 22 to April 7. You can get involved by simply giving up a favourite treat such as chocolate during Lent, putting the money you save into a Wee Box, and then donating it to SCIAF.

The charity also wants people to sign SCIAF's latest campaign action calling on the UK Government to implement a "Robin Hood tax" – a small tax on the financial sector to help raise the billions needed to tackle global poverty.

To order your Wee Box and sign the campaign action visit www.theweebox.org or call 0141 354 5555.