The Candys' make-a-pile driver

It is London's most expensive building site, a mini-village of hard-hatted construction workers that has condemned Knightsbridge to traffic hell for months.

But this week One Hyde Park - the flagship development of billionaires Nick and Christian Candy - finally turns the corner in its slow progress from muddy hole in the ground to superexclusive serviced apartment blocks.

The last of 603 piles sunk into the London clay on Friday will mean that the foundations of the four towers - or "pavilions" as the Candys call them - are complete.

This first stage has proved tougher that expected because of the volume of concrete that had to be removed from Bowater House, the ugly Fifties office block previously on the site.

Nick Candy told the Standard: "There was a lot more concrete in the ground than we thought, around 50 per cent more. But there was absolutely no way of telling from the old drawings until we actually started digging."

The Monaco-based brothers say the problems have not held back the project, which is "a few days" ahead of schedule. The completion of the foundations, which have been dug down as far as 57 metres, triggers another payment from the 40 purchasers already committed to buy one of the 80 apartments.

Prices range from £6 million to a reported £100 million - reputedly to Sheikh Hamad, foreign minister of Qatar - and average £19 million.

With £750 million worth already sold, the Candys say they have enough coming in to pay off the banks and keep shareholders happy. The brothers argue that although sales have slowed to around two a month, the credit crunch has not hurt their target market. "If you are a billionaire or a multi-billionaire, what you are paying is just interest on interest, it makes very little difference.

"If we sell at this rate it will be 24 more this year and that leaves just 16. We have been very strict about who we want, we have turned away people who are not suitable."

The brothers say most of the 1,125 dollar billionaires in the latest Forbes magazine have taken an interest. The Candys this week entered the Sunday Times Rich List for the first time with a fortune estimated at £120 million. The nine-storey "Pavilion A" closest to the Mandarin Oriental Hyde Park will be complete by the end of the year. The others will be finished by next July with the fitting out and internal detail complete by the end of 2010.

The project is been swathed in secrecy. An unnamed member of the royal family hosted a reception on the site in the last few weeks but very few other " outsiders" have been near it.