Author Ian Sayer, 61, has been tracking down stolen Nazi   gold and war criminals for the past 30 years. Along the way, he has acquired some valuable second world war documents and signatures.

In 1984 he wrote the international bestseller Nazi Gold – The Story of the World’s Greatest Robbery which described the 1945 looting of the Nazis’ gold and foreign currency reserves by a consortium of former German and US officers from its hiding place in the Bavarian mountains.

As a result of his research for this book, and Hitler’s Last General – his successful search for the world’s most senior surviving war criminal, SS General Wilhelm Mohnke – Sayer has become a leading authority on documents relating to the second world war.

He says: “I collected this material because of its historical significance, but in the process the commercial value of much of it has far outstripped the stock market.

“Some of the more important documents have been exhibited in displays at various museums, including the Imperial War Museum in London. Many items have been loaned to numerous researchers and authors.”

Sayer explains that anyone with a good command of the German language will be at a considerable advantage. “There tend to be far more German documents on the market than American or British, as many of them were picked up and taken by the Allied troops as war booty.”

Mentioning a signed photograph of Churchill that he reckons is worth a couple of thousand pounds, the historian says: “Twenty-five years ago you could have picked it up for between £50 and £100. Depending on the quality, a signed photograph of Churchill may be worth anything from £1,750 up to £15,000, but there are quite a number of forgeries around.”

After Sayer left school at 16, he worked for several years in the insurance and airfreight industries. At 20 he started a transport company, and became a pioneer off next-day delivery services in the UK, eventually selling out to an Australian company in 1979. Since then he has been a transportation consultant and a director of various companies.

He says: “I sold my company for £3.5m, which enabled me to pursue my personal interests of researching, writing and investigating various aspects of World War Two. I began collecting historical documents, letters and photographs in 1978.”

The author has some fascinating insights into the values, authenticity and availability of historical autographs. “A Mussolini signature is not worth as much as it used to be, as a lot of documents flooded the market about 20 years ago, depressing the value.

“Before the Iron Curtain came down in 1989, documents signed by Stalin were difficult to get hold of and very expensive, perhaps as much as £5,000 each. Now, with more material on the market, anything signed by Stalin is worth £2,000-£3,000.

“A couple of letters written in 1940 by Ann Frank sold at auction in New York in 1988 for £75,000. The only Oscar Schindler signature I have come across sold for around £3,000 at a New York auction just a few years ago.”

Autographs and documents can rise dramatically in value if a film is made about a particular event or war hero. The story of Colonel Claus Von Stauffenberg, the leader of the July 1944 conspiracy to assassinate Hitler, is being made into a film starring Tom Cruise as Claus. The film’s title, Valkyrie, was the plotters’ code name for the operation. When the conspiracy failed, Claus was arrested and executed. A signed document of his is worth £3,000-£4,000 but there were other plotters.

Sayer believes that after the film comes out the price for any Stauffenberg signature will go up by at least £1,000. “More people will be keen to acquire one, and signatures of the other plotters will rise accordingly,” he says.

The historian has amassed his collection from a variety of sources but the vast majority of pieces have come from private collections and auctions.

However, inaccurate descriptions, even from well-known auction houses, can provide a trap for the uninitiated. Sayer relates a story that took some years to unfold.

At the conclusion of the second world war in Europe, the Allied Commander-in-Chief, General (later President) Eisenhower issued a stark message to his senior staff. Classified “Top Secret”, this telegram simply stated: “The mission of this Allied Force was fulfilled on 0241, local time, May 7, 1945.” Some of Eisenhower’s staff requested him to sign their copy of this historic message.

In July 1980 the first of these signed copies to come on the market was auctioned at Sotheby’s in London. Sayer says: “The estimate was £300-£400 but it sold for £1,600. I dropped out at £1,500, thinking I might come across another at a cheaper price. After the auction I regretted that I had let it go.”

Less than three weeks later, Sayer discovered that the original message was to be sold by New York auctioneers Charles Hamilton Galleries. This document, the description emphatically claimed, was the original and not a copy. The estimate was $35,000-$40,000.

Sayer says: “After the disappointment at Sotheby’s, I decided to go for it. However, it turned out that I was bidding against Malcolm Forbes, one of the richest men in America. Again I dropped out, this time at $44,000, and it went to Forbes, for his museum, at $48,000.

“Three months later, Charles Hamilton wrote to me and offered a copy of the Eisenhower message (Number 34) for $16,000. While I was considering the offer I looked again at the catalogue illustration of the original message in the earlier sale.

“It was only then that I noticed the partly obscured words “Copy No 16” which clearly indicated to me that the ‘original’ was merely another copy. I wrote to Charles Hamilton Galleries and challenged them on this point. I did not receive a satisfactory response, so passed on Copy No 34.”

In 1985 Sayer learned of a private advert in the European edition of the New York Herald Tribune, offering another copy of the message for sale.

“I contacted the vendor in Germany. This copy turned out to be the one that I had failed to buy at Sotheby’s in July 1980. Five years later I ended up buying it for £10,000,” he says.

The story does not end there. In March 2002, following the death of Malcolm Forbes, Christie’s in New York was appointed to dispose of many of his museum’s second world war exhibits, including the Eisenhower “original” message that Forbes had acquired in 1980 from Charles Hamilton Galleries.

The Christie’s catalogue description replicated the Hamilton description and the message once again purported to be the original. It was not. Sayer says: “In fact it was just Copy No 16, but the purchaser, who paid $140,000 thinking it was the original, is still unaware of the true facts.”

In 1981 Sayer tracked down the US Army major, Ruth Briggs, who had typed the telegram. Eisenhower had written the message in longhand, Major Briggs had typed it up, then thrown the handwritten original into the wastepaper basket.

Realising the importance of the document, she immediately retrieved it.

“When I asked her what had happened to it she did not respond!” says Sayer. “Today, that handwritten note might make upwards of $250,000 at auction.”