Midget submarine that sunk a 10,000-ton Japanese cruiser and won two Victoria Crosses

The mural painted at the end of a block of flats in East Belfast is some 36 feet high and 24 feet wide. It depicts a favourite son of Belfast, the only man from Northern Ireland to be awarded the Victoria Cross for bravery during the Second World War. The image of Leading Seaman James Magennis shows him as a young man dressed in his Royal Navy uniform, proudly sporting his VC on his chest. There is a row of remembrance poppies painted at the base of the mural, while the only words on the wall are: "James Magennis VC", "Born 1919" and "Died 1986".

I travelled to Belfast days ahead of the 80th anniversary of Magennis' VC action, which falls on July 31, to tell the story of his remarkable life and outstanding courage - valour that saw him save the lives of four men - including himself - beneath the waves in a mini-submarine that was carrying out a clandestine mission behind enemy lines.

His gallantry award also has great personal significance to me because, nearly 40 years ago in 1986, it was the first VC medal group I purchased. Little did I know then I would go on to build the world's largest collection of VCs - currently more than 200 in total.

James Joseph Magennis was born into a working-class Roman Catholic family from West Belfast on October 27 1919. His father, William, was a musician and part-time packer in a mill, while his mother, Mary, gave birth to two other sons. Magennis enlisted in the Royal Navy in June 1935, aged just 15.

The Second World War began in 1939 and it was not long before he showed an aptitude for bravery. While serving in the K-class destroyer HMS Kandahar, he was involved in successful action on June 23, 1940, against the Italian submarine Torricelli. After the submarine had been hit by the British ship, Magennis jumped into shark-infested seas to help save Italian servicemen.

Later, Magennis was serving during the evacuation following the Battle of Crete when he and another sailor repeatedly dived into the sea to rescue men - this time Allied personnel, who were in danger of being killed by German Stuka dive bombers after their ship's sinking.

On December 19, 1941, Kandahar was mined near Tripoli, Libya, and the crew were forced to abandon ship with the loss of 67 lives. As one of the survivors, Magennis was evacuated via Alexandria, Egypt. In 1942, he began submarine training and the following year he volunteered for "special and hazardous duties", training as a diver.

In 1944, Magennis registered for service in XE-class, or midget, submarines. Then in late July 1945, only days before the atomic bomb was dropped on Hiroshima, Lieutenant Ian Fraser, Magennis and two other sailors were ordered to take part in an attack on the Japanese navy. Their task was to sink the Takao, a 10,000-ton Japanese cruiser that was moored in the Johor Straits, off Singapore. To get into position, however, they would have to negotiate treacherous seas - and then slip out unnoticed before their explosives detonated.

Fraser was in command of the mission, while Leading Seaman Magennis was the diver who would have to attach the limpet mines to the hull of the enemy ship. Their midget submarine, XE.3, was towed to the area by a conventional submarine, HMS Stygian, before slipping its tow at 11pm on July 30. Ahead was a perilous 40-mile journey through wrecks, minefields, listening posts and surface patrols.

Conditions in the midget submarine were cramped, stuffy and uncomfortable. For 13 and a half hours, their journey went without any hitches so that by 12.30pm on July 31 their target was in sight. At 3.03pm, Fraser slid the midget submarine almost under Takao, with the enemy ship anchored in shallow waters less than 100 yards from shore.

Magennis, by this point aged 25, emerged from the "wet and dry compartment" of the submarine which could either be flooded or pumped dry to let a diver exit or re-enter the submarine. Soon he had begun his task of attaching limpet mines to the enemy ship. However, he had to chip away at barnacles in order to attach the magnetic explosives.

Eventually, despite a leak in his oxygen line, Magennis attached six limpet mines to the hull before returning, exhausted and with shredded hands, to the sub.

However, on a falling tide, the craft became wedged beneath Takao. For more than an hour, Fraser tried to break free but to no avail. It looked as if the mini-submarine and her crew would be blown to pieces by her own explosives as there were only six hours before the charges went off.

Suddenly, however, the midget submarine shot backwards. It was no longer trapped but a new problem had arisen. When they tried to release the heavy limpet mine carrier from the sub's exterior, it would not break free and they could not drag it along on their 40 mile-journey to safety. Knowing Magennis was injured, Fraser volunteered to dive to free it but Magennis insisted that he was the more experienced diver. "I'll be all right as soon as I've got my wind, sir," he told Fraser.

Magennis slipped into the water again clutching a large spanner and, after struggling for seven minutes, managed to release the charge. Once safely back in the submarine, the men began their journey back at full speed to safety.

Magnennis later said of this phase of the operation: "The water was muddy but when I looked closely I saw the lifting clips had not fallen away. I unclipped one, then the other. The midget submarine started to float away. I was still astride the side cargo. I had to swim for my life. Luck was with me. I made it.

"When I drained down in the wet and dry compartment and climbed back into the main section of the submarine, I was barely conscious. I remember Lt Fraser saying, 'You're a gem, Mick [his nickname].'"

Eventually, the crew made a successful rendezvous with Stygian and was towed to safety. The men had been 52 hours without sleep other than a short doze just before the attack. Furthermore, "Operation Struggle", as the mission was appropriately codenamed, was a success: their charges blew a giant hole in the enemy ship's hull.

The two sailors were awarded the VC on November 13, 1945, shortly after the war had ended. Magennis's citation concluded that he had "displayed very great courage and devotion to duty and complete disregard for his own safety". The two other crew members were decorated with lesser gallantry medals.

On December 11, 1945, the two men were presented with their VCs by George VI in an investiture at Buckingham Palace. "Well done, my lad," the King told Magennis. Citizens in Belfast organised a "shilling fun" for Magennis that raised more than £3,000 for him - a considerable sum 80 years ago.

Magennis married Edna in 1946 and the couple went on to have four sons. He was discharged from the Navy with the rank of Leading Seaman in 1949, by this point living back in Belfast. For a time, he worked at Royal Navy Air Station Sydenham, Belfast.

However, Magennis did not cope well with the fame that his VC brought and he became too fond of yet another celebration drink. The family suffered a tragedy in 1952 when one of Magennis's sons, David, was killed in a car accident. Impoverished, soon afterwards he sold his VC medal group to a Belfast dealer for £75. Once the sale of the medal became known, Magennis was widely criticised, even vilified, for his actions. Eventually, the dealer returned the medals to Magennis amid rumours he had been paid to do so by a wealthy intermediary.

With sectarian tensions growing in his home city, Magennis was in a difficult position as a Catholic who had fought for what was becoming, in the West Belfast Catholic strongholds, at least, the much-hated British Armed Forces. It was almost certainly for this reason that he chose to live in East Belfast, traditionally a Protestant area.

With these challenges in mind, it was perhaps not surprising that Magennis and his family moved to Yorkshire in 1955, where he initially lived near Doncaster and worked as an electrician, having long since squandered any riches that he once had. Magennis died from lung cancer, while living in Halifax, on February 12, 1986, aged 66. He was cremated at Nab Wood Crematorium, Shipley, Yorkshire.

Over time, both sides of the religious divide in Northern Ireland have come to acknowledge Magennis' courage. A memorial to Magennis was unveiled in the grounds of Belfast City Hall in 1999. Six years later, the mural commemorating Magennis was unveiled in the city on the 60th anniversary of VJ-Day.

When Magennis' gallantry and service medals came up for sale in 1986, I was able to fulfil a long ambition to purchase a Victoria Cross, Britain and the Commonwealth's most prestigious decoration for gallantry in the face of the enemy. The memorabilia that came with the medal group included the diving suit that Magennis had used for his VC action.

Two years later, in 1988, I took advantage of the opportunity of buying Fraser's VC, thereby "uniting" the two decorations.

Leading Seaman James Magennis VC, despite all the complexities that surrounded his birthplace and parts of his personal life, was a truly courageous individual. I, for one, will always do my best to champion his bravery far and wide.

• Lord Ashcroft KCMG PC is an international businessman, philanthropist, author and pollster. For more information on his work, visit lordashcroft.com Follow him on X/Facebook @LordAshcroft