Image copyright Publicity Image caption The BBC only ever leased the building: it never owned it and now it's been turned into a modernised version of how it was built in the 1920s by New York businessman Irving T Bush

For 70 years it was home to the BBC World Service. But the broadcasters have gone and Bush House in London has become part of the new Aldwych Quarter.

A £61m refurbishment has ripped out the characteristically BBC warren of studios and shabby offices and created a centre for modern business.

But that, as the architect in charge of refurbishment points out, was what Irving T Bush always intended.

You could spend hours with architect John Robertson striding around the enormous office space he's just refurbished in central London without exhausting his enthusiasm for the former headquarters of BBC World Service radio.

The studios are all gone but almost every stairwell or basement has some tiny detail of design to draw his attention - from an intricately wrought banister rail to an elegant piece of coving which for decades lurked hidden above a false ceiling.

Robertson was commissioned by the Japanese owners, Kato Kagaku, to return Bush House to a modernised version of how it was built in the 1920s by New York businessman Irving T Bush.

Image caption Architect John Robertson says the whole intention of the work on Bush House was to strip out the BBC infrastructure and go back to something like the original floorplan

Bush, working with US architect Harvey W Corbett, wanted a grand international business centre to match the one he'd opened in Manhattan in 1918. The Bush Terminal Sales Building still stands on 42nd Street, known today as Bush Tower.

Bush House in London opened in stages from the mid-1920s. Twenty years later the BBC moved in, initially to take over a radio studio, belonging to advertising company J Walter Thompson, needed for transmissions in wartime. Bit by bit the BBC colonised the vast complex.

The World Service finally quit its shabby but much-loved base on the Aldwych in late 2012, moving to New Broadcasting House on Portland Place.

John Robertson says legal firms and advertising and media companies are among those likely to move into the rejuvenated Aldwych Quarter. The name Bush House is now reserved for what generations of BBC staff knew as Centre Block.

Image copyright Hufton and Crow Image caption Aldwych Quarter is a sequence of pristine, white-walled open-plan spaces

The other buildings have been rebranded Strand House, King House and Melbourne House. Robertson says it's possible a single tenant could take the entire development but it may be that each block is leased individually.

The building work is virtually complete except for the large basement space which formerly contained the BBC canteen. "For now we're leaving that as a shell until it's clear what the new tenants want. The Aldwych area is changing fast and there might be room for an up-market restaurant there in a way no one would have foreseen a few years ago."

The prospect of a swanky new brasserie emerging under Bush House will amuse those who knew the BBC staff canteen in its latter days.

For now Aldwych Quarter is a sequence of pristine, white-walled open-plan spaces. It is almost impossible to recall the journalistic clutter which built up over the decades of BBC occupancy. The former Centre Block is Grade 2 listed, meaning nothing could be changed without detailed negotiation.

John Robertson says the whole intention was to strip out the BBC infrastructure and go back to something like the original floorplan. "So even in the other three blocks there's very little we changed.

Image copyright Hufton and Crow Image caption John Robertson thinks the building is hard to classify architecturally but says he calls it an American take on the neo-classical Beaux-Arts style

"With any redevelopment like this you now have to put in a raised floor for cabling and wi-fi. And 90 years on people expect air conditioning and more efficient heating. So we've brought the building back to life for today. But if Irving Bush and Harvey Corbett could wander through the place today it wouldn't seem that different to what they built almost a century ago. They wanted flexibility which is exactly the intention today as well."

"The stripping-out stage of the project was extraordinary. It was clear the BBC had never taken anything out - it just added more and more kit year by year. So what you might call the demolition stage took almost as long as the refit as such. The amount of wiring to come out was immense: we spent £7m on just taking out the asbestos and copper cabling.

"One of the glories of the interior has always been the travertine marble walls and flooring. In the lobbies we had to do surprisingly little patching up to do and I like the fact that the marble has a certain patina which came with age."

Harvey Corbett, born in San Francisco, was also one of the architects who worked on the Rockefeller Center in Manhattan. The public areas of Bush House have always had a slightly American feel.

Image copyright Hufton and Crow Image caption John Robertson: 'It may be a bit fanciful but when you think of the amazing collection of people who came here, I like to think there's still a few ghosts hanging around'

John Robertson thinks the building is hard to classify architecturally but says he calls it an American take on the neo-classical Beaux-Arts style. "There are Art Deco elements too, as in a lot of commercial buildings of the 1920s and 30s. Details like the brass dry-risers are like something out of 1920s Manhattan.

"Even if those objects hadn't been listed we'd still have catalogued them and refurbished them carefully. Sometimes it's just a light-fitting but cumulatively small things add so much to the feel of a building. That's why we improved the look of the very unusual internal post boxes with their brass chutes dropping from floor to floor down to a central collection post. The system's not in use but it gives character to the building."

The corporation only ever leased the building: it never owned it. Robertson says the whole project, which at one point employed almost 1000 people, has been focused on returning the complex to its Bush-Corbett origins, not on evoking the long presence of the BBC.

"But the typeface used throughout the building to mark floor numbers and so on is Gill Sans - it was very a deliberate choice as it's the font the BBC uses for its logo." Otherwise, the architect says, there's little to remind its new tenants of its BBC days.

Ironically one of the best-known features of Bush House, which people always associated with broadcasting, wasn't a BBC creation at all.

Above the imposing front door stands the ringing dedication - To the Friendship of English Speaking Peoples. John Robertson says it was part of the original pre-BBC design.

"Irving Bush and Harvey Corbett saw the building as a great centre of international commerce. In fact it never quite happened, partly because of the economic depression of the 1930s. Then the BBC moved in and somehow took over. So in a way the building is returning to its original purpose.

"One of the things I love is the echo in the lobbies. It may be a bit fanciful but when you think of the amazing collection of people who came here, I like to think there's still a few ghosts hanging around."