Cheryl Fawley knows that if she softly places her hand on John's cheek he will sip his water. John needs to drink a lot because the 48-year-old, who can't talk or walk and is paralysed down one side of his body, is diabetic. But for some reason – perhaps the military-style operation required for him to use the toilet – John can be reluctant to take in liquids.

"You need patience. He likes to be comforted. If you just gently put your hand on his cheek, he'll drink," confided Fawley. It's one of the many pieces of wisdom – trivial, and yet not – that this slight, nervous mother-of-three has picked up over her 16 years as a support worker looking after people in their homes who have been stricken by disability, disease and frailty.

On Friday, though, Fawley couldn't be by John's side. She was picketing with colleagues and friends outside the imposing, grey-tinted windows of the four-storey central London headquarters of Bridgepoint Capital, the private equity firm that has a majority shareholding in the healthcare company Care UK.

Fawley, 42, is one of 237 support workers who were transferred from the NHS to work for Care UK when it won the contract to run community care services for people with learning disabilities in Doncaster. And she is one of the 50 who are on strike, fighting pay cuts of up to 35% imposed by the private health company and the £7-an-hour wage paid to the 100 new staff replacing some of those who have walked away in disgust.

This determined, indignant group of strikers will have withdrawn their labour for 48 days in all, with gaps in between where arbitration and a court battle against the imposition of the new terms failed.

The strike will go on, though. Perhaps for many weeks. It's looking likely that this could be the longest strike the health service has seen. "I expect the strikers to support more action, and for this to intensify," said Jim Bell, the Unison organiser.

Fawley and her colleagues believe they are fighting for a fair wage. Her £8.91 an hour used to go up to nearly £12 when she worked through the night helping John and others. It would go to around £14 an hour on a bank holiday or weekend. It wasn't a fortune, and it involved time away from the family, but an annual income of £21,000 "allowed us a life", she says. Care UK ripped up those NHS ways when it took over.

Today Fawley, like her new colleagues who are on £7 an hour, receives an extra £1 an hour for a night shift and £2 an hour for weekends. Perhaps it shouldn't, but it smarts with some of the strikers when they hear that the highest paid director of Bridgepoint, Care UK's owner, received a salary of £1.1m in 2012, or that £14m was passed around its staff in bonuses.

But Fawley isn't into envy. She loves her job. Loves the people she works with, and says for her this battle is as much about looking after the interests of the people she and others care for, here and elsewhere around the country, where firms such as Care UK are increasingly seeking to take over this type of service.

"It's like we have had all these years building up experience, learning from our mistakes, and now it is all meaningless," she said. "The NHS encourages you to have these NVQs, all this training, improve your knowledge, and then they [private care companies] come along and it all comes to nothing."

Some of the striking health workers from Doncaster. Photograph: Richard Saker for the Observer

Fawley's husband Jason, 41, who works in a warehouse for Tesco, admits that recent months have been hard, and the pressure has told on their relationship at times. They face losing £400 a month – a quarter of their income. His wife's annual holiday has also been reduced by eight days and, should she fall sick, the first three days off work will now be unpaid. "She is up at night worrying about paying the bills," he said. "People are going to have to leave. They are bringing in people on £7 an hour who often don't want to do this sort of job. You have to pay the bankers their bonuses to keep the talent. Is this not as important or more important – keeping these people safe?"

Chris Hindle, director of learning disabilities at Care UK, insists that his employer is not in the game of "extracting huge profits" and that Doncaster council set the ceiling for their budget for the service. He said that the NHS would have had to make the same sort of changes if it had carried on with the service, because the council's budget was £6.7m over three years and the wage bill alone stood at £7m.

The former NHS provider of the service, Rotherham Doncaster and South Humber NHS Foundation Trust, isn't sure of that claim. Changes to terms would have been made, but "we would have worked in partnership with staff colleagues and their union representatives to agree the minimum changes required to ensure we could carry out the service for the fee that was being paid", a spokesman said.

For all the talk of this private provider being forced by circumstances to make difficult decisions, Care UK expects to make a profit "of under 6%" by the end of the three-year contract it has for running the service, Hindle admits.

Accounts show that, in conjunction with another service run in Lancashire, Care UK's learning disability services delivered a £700,000 operating profit in the six months between September last year and March this year, although Hindle insists that they are now running at a loss. However, in modern Britain, care is certainly big business and it is getting bigger. The changes in Doncaster have been going on elsewhere for some time, but perhaps without a similar level of resistance.

In 1993 the private sector provided 5% of the state-funded services given to people in their homes, known as domiciliary care. By 2012 this had risen to 89% – largely driven by the local authorities' need for cheaper ways to deliver services and the private sector's assurance that they could provide the answer. More than £2.7bn is spent by the state on this type of care every year. Private providers have targeted wages as a way to slice out profits, de-skilling the sector in the process.

According to the thinktank the Centre for Health and the Public Interest, about 1.4 million care workers in England are unregulated by any professional body and less than 50% have completed a basic NVQ2 level qualification, with 30% apparently not even completing basic induction training.

Yet the big hitters in private equity know there is money to be made for those at the top. Such firms – using debt to purchase health companies with a toe in the market, and often using those health firms as cash cows to pay off that debt and replenish the private equity firms' coffers – need cash generation to make an investment in health work, and they have been moving in on health and social care in a big way.

Today 8% of care homes are supplied by private equity-owned firms – and the number is growing. The same is true of 10% of services run for those with learning disabilities. William Laing, an economist whose consultancy, Laing & Buisson, compiled a report on private equity in July 2012 for the British Private Equity and Venture Capital Association, puts it bluntly: "It makes pots of money." Those profits – which are made before debt payments and overheads – don't appear on the bottom line of the health firms' company accounts, and because of that corporation tax isn't paid on them.

The debt used to buy the health firms is set against the profits, often via big intercompany loans, with huge interest rates to match. Care UK, while currently posting operating losses of £9m, paid £90m in interest payments in 2012-13, the last year for which accounts are available. Some of that was in payments on loans issued in Guernsey, meaning tax could not be charged. Its sister company, Silver Sea, responsible for funding the construction of Care UK care homes, is domiciled in the tax haven of Luxembourg. Care UK's owner, Bridgepoint, whose other acquisitions include Oasis Dental, Pret a Manger, and the clothing retailer Fat Face, knows there is money to be made. There are "excellent growth prospects and consolidation opportunities for private-sector players that can offer flexible, efficient and innovative business models in this evolving environment", its website says. It remains to be seen if such an evolution will also deliver what Fawley fears: an underpaid, de-skilled workforce, fresh from the jobcentre, whose patience with the Johns of this world will wear thin.