Paul Taylor skipped “A” in the A-Z careers guide – accountancy and actuarial jobs did not excite him – so he and one of his school friends opted for B and banking. His friend joined Barclays and Taylor became a cashier at the East Sheen branch of NatWest.

Taylor rose through the ranks and ended up spearheading NatWest’s structured-finance business. Then he quit to become right-hand man to Robert and Vincent Tchenguiz at their sprawling Rotch property empire. Today Taylor’s school friend is an officer in the Metropolitan Police and Taylor is the UK and European investment adviser to the Gulf emirate of Qatar.

During the past decade Taylor has been forging a strong relationship with the hugely wealthy Qatar Investment Authority. And 12 months ago he decided to create his own business to advise it on its British and continental investments. Since then he has built up a £3.5 billion investment portfolio for the Gulf kingdom. It includes Four Seasons, Britain’s biggest nursing-home company, other property assets and Senad, an education company that specialises in teaching children with severe learning difficulties. Combined, the companies employ 50,000 people in Britain.

But it is the 25% stake he has amassed in J Sainsbury that has turned Taylor into one of the most talked-about and intriguing private businessmen in Britain.

The stake-building has led to speculation that Taylor is plotting a full-blown bid for the supermarket giant which, with a market value of £10 billion, would make his other investments look like chicken-feed.

During the past six months he has shunned media attention, but this week Taylor spoke to The Sunday Times. He remained tight-lipped on the takeover talk – there are strict City regulations on who can say what in potential takeover situations – but he spoke publicly for the first time about why he had bought the stake.

“Justin King [the Sainsbury chief executive] has a long way to go. It’s a good story. Sainsbury has always been an aspirational brand. It used to be the meal that mum would aspire to put on the table once a week. It’s motto used to be ‘good food costs less at Sainsbury’ – they dropped that, unfortunately, but it’s something that I can chime with,” he said.

Given Taylor’s background in property, working with the Tchenguiz brothers, the City has been speculating that his interest lies in the supermarket’s £8 billion plus property portfolio rather than its core operating business. After all, market watchers say, his former colleague Robert Tchenguiz has amassed a 5% stake of his own in Sainsbury and drawn up proposals to spin off the property into a separate vehicle to crystallise its value.

Taylor is adamant that his strategy is different. “It is not about real estate for me. It is about investing in good, solid fundamental UK businesses.”

He likes the Sainsbury management team, too – although he has not met them in person yet. “Justin is capturing the mood of good quality at better value. From what we see, we are great supporters of him,” said Taylor.

Taylor’s company is named Three Delta after the classroom at his old school because his headmaster said that nothing good would ever come from Three Delta. “He is still alive and if he is watching I am trying to prove him wrong,” laughed Taylor.

Taylor described Three Delta as a private capital investor – coining the term to distinguish the group from conventional private-equity firms.

“It is not like private equity for a number of reasons – it is my own money and their [the Qatari Investment Authority’s] own money. When you are spending your own money, like Sir Philip Green, Robert Tchenguiz or Sir Richard Branson, you have a different attitude to investment.”

Taylor stressed that his partnership with the Qataris allows him to invest for the long term. “We are not in a rush to spend money. We don’t have sunset periods on our funds which mean they get taken away if they don’t get spent. We don’t have any preordained horizons,” he said.

“It’s not driven by IRR [internal rate of return, used to decide whether firms should invest for the long term]. We want to get it right. We are sophisticated investors – it is important to me and to the country,” he said.

In contrast to many private-equity firms and other private companies that do not disclose the names of their investors, Taylor points out that the origins of the cash at Three Delta are clear.

“It’s obvious where the money comes from. The Qataris have gas reserves that will go on for 250 years,” said Taylor.

He described Three Delta as a “partnership between me and the country. I make the decisions. They are passive investors.”

Every day Taylor is in close contact with either Sheikh Hamad bin Jassim bin Jabr al Thani, the Qatari prime minister, or the country’s finance minister. “We converse every day, but it is not a formal arrangement. It is a relationship that’s very personal and it works,” he said.

The structure of the Three Delta group is comparatively straightforward. The Qatari Investment Authority invests in Delta Commercial, an Isle of Man-based vehicle, which owns most of the partnership’s investments in Britain.

Three Delta – Taylor’s British-based company – then acts as adviser to the Delta Commercial partnership. The stake in Sainsbury is held through a separate, special-purpose vehicle called Delta II, which Three Delta also advises.

The Three Delta office in London’s Hanover Square, plus its 20 or so staff drawn from the banking and property world, are all funded from Taylor’s pocket. He also co-invests in the partnership with Qatar. Taylor has ambitions to expand the business beyond Britain and has a mandate to invest across mainland Europe.

For now he has his hands full, but he wants to start building a network of European offices next year. Taylor hopes to expand Three Delta’s tentacles into new investment areas – housebuilding and financial services seem to be at the top of his shopping list.

How big will the company grow? “We have the scope to be substantial – and I mean really substantial,” said Taylor. It is not surprising then that a number of banks are working very hard to build a close relationship with Three Delta. It is a company that is planning for the long term and has the finance to achieve that goal.

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