Six years ago, when Nick and Christian Candy started to build One Hyde Park, they had confidence that it would become the most expensive, exclusive apartment block in the world. The subsequent global economic crisis turned the project into a huge gamble.

Project Grande (Guernsey), their joint venture with a company owned by the prime minister of Qatar, needed to take out a £1.15bn loan from the German bank Eurohypo AG to pay for the land in London's wealthy Knightsbridge area and meet construction costs to deliver the promised "ultimate perfection".

But the economy was booming, credit was cheap, and in 2007 and 2008, sales were in line with expectations – or better. A penthouse was sold to Rinat Akhmetov, reportedly Ukraine's richest man, for a record-breaking £136m.

Then the credit crunch hit. In the words of Liam Bailey of Knight Frank, the estate agency selling the apartments, "global property markets didn't just seize up, they looked deep into the abyss".

In 2009, many wrote off the project as an ego-trip gone wrong and the company took the unusual step of removing the apartments from the market to wait for better times. They were put back on sale in 2010 and officially launched this April at a lavish party at the next-door Mandarin Oriental hotel, with a glittering guest list which included Bernie Ecclestone, Andrew Lloyd Webber and Gary Lineker. Developer Nick Candy, 38, attended with his girlfriend, the singer and actress Holly Valance, 28.

Now analysis of the finances of the giant glass-and-concrete block of flats, sandwiched between Harvey Nichols and the Serpentine, reveals that the Candy brothers' gamble has paid off. A letter from the developer's lawyers, SJ Berwin, obtained by the Observer, reveals that the project has "fully discharged and repaid its £1.15bn development facility with Eurohypo AG".

Records held by the Land Registry show that 60 apartments have been sold – and that one of the parking spaces fetched a staggering £250,000. Three units of storage space sold for £150,000, £157,000 and £50,000. The company says a further two apartments have also been sold but are yet to show on the records, bringing total sales to £1.424bn.

A branch of the Abu Dhabi Islamic Bank will open its doors in the building, alongside Rolex and a showroom for McLaren cars. Planning permission was granted for 86 apartments, but the final number is expected to be 80 because some buyers are merging flats. Eighteen apartments are still to sell, but with the loan paid off, the Candy brothers need not lose too much sleep.

So who is buying up One Hyde Park and why? The developers believe the Arab spring has spooked the super-rich from the Middle East into seeking less risky investments in the UK and they also claim to have been "most recently experiencing sales interest from investors in the eurozone looking for a safe haven for their money". London, it appears, is where the world's super-rich are turning to protect their cash.

Some of the few identities of One Hyde Park purchasers that can be discovered from Land Registry records include Mohammed Saud Sultan al-Qasimi, head of finance for the government of Sharjah, one of the United Arab Emirates, who spent £11,493,700 on an apartment. Insurance entrepreneur Rory Carvill spent £21,215,000 on a property plus additional storage space.

Christian Candy has two apartments, which cost him £26.2m and £31m. Raymond Grehan, an Irish property magnate, paid £3.75m for his purchase which he is now trying to sell for £5.6m.

The records reveal extensive use of tax havens to purchase the properties with the British Virgin Islands, involved in 25 deals, the most popular choice.

Critics of tax avoidance by the super-rich will point out that an estimated £750m a year has been lost nationally in duty due to the use of offshore vehicles. People who purchase the flats from the current owners can avoid stamp duty by buying the offshore companies used to buy them without triggering a taxable property transfer.

Another concerned party, the Observer has learned, is Westminster city council which has launched an investigation into the current owners' identities after discovering that only nine apartments have registered for council tax, five of which are for second homes.

Council officials wrote to the developers asking for all the owners' names two weeks ago but are yet to receive a response. They may finally have to go door-to-door to claim the annual tax for each apartment of £755.60, plus £619.64 for the Greater London Authority.

But the Candy brothers say all the buyers paid their stamp tax at the time of purchase, and they are not responsible for registering properties for council tax. Nick Candy said his hopes continue to be high for the development. "I am very confident of it as a solid investment for the future, especially in 2012 when the eyes of the world will be on London."