If Amelia Morris wants to visit her nearest Marks & Spencer store, the American fund manager has to travel 5,000 miles from her home in San Diego, California to Falmouth, Cornwall.

But despite the distance, Morris, a mother of four, managed to spot what many of the City’s most highly paid analysts and fund managers missed: the intrinsic value of the British high-street icon.

Since April 1999 Morris’s firm, Brandes Investment Partners, has invested more than £700m in M&S. After nearly six years her faith has paid off. In the past three months M&S shares have soared 43%, touching a seven-year high last week.

Brandes is now sitting on a paper profit of £500m, according to analysis of stock-market filings by The Sunday Times. For the firm it is a vindication of its value-investing philosophy. For Morris, a senior analyst at Brandes, it is a personal victory.

While analysts question whether the M&S recovery is really sustainable and traders ask how much higher the share price can go, Morris can relax in the knowledge that the value of her investment has almost doubled.

The wife of a former Wall Street banker — Ken, who now writes financial thrillers — Morris has invested billions in the UK high street over the past five years. As well as her £1.2 billion holding in M&S, she owns a combined £1.3 billion stake in Wm Morrison and J Sainsbury as well as smaller investments in HMV and French Connection.

Brandes has also bought shares in more than a dozen other British companies, including Invensys, the engineering firm, Compass, the contract caterer, and Corus, the steel group.

Morris visits Britain three or four times a year: a tennis-mad, slim woman with dark hair and striking blue eyes, she plans her trips to coincide with Queen’s or Wimbledon. She is also a regular at the Deutsche Bank retail conference in October.

Yet both Brandes and Morris shun publicity: they will not talk to the press, while those who have met Morris refuse to speak publicly about her. “She doesn’t rush into investment. She does a lot of homework,” said one chief executive.

Some describe her as closed and not very personable. “She is single-minded with a long-term focus. Very bright, but very quiet. You could mistake her for a schoolteacher from the Midwest,” said another chief executive.

But Morris was reluctantly thrust into the spotlight during Philip Green’s £9 billion bid for M&S, when she urged the board to allow the entrepreneur to carry out due diligence. It was an unusual intervention by the American firm, which has never been an activist shareholder.

In the reception of the Brandes headquarters building is a copy of the 1949 investment classic, The Intelligent Investor, by Benjamin Graham. Charles Brandes, the founder of the firm, boasts about a 100-year outlook: “We don’t have an economist. We will never have an economist. We don’t invest in the economy. We invest in businesses.”

Brandes’ investment decisions are based on a company’s cashflow, book value, market capitalisation and other indicators. It is a philosophy that gave Morris the confidence to buy and sell (at opportune moments) £1.8 billion worth of M&S shares over the past six years.

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