It’s like meeting an old friend you haven’t seen in a very long time. You are damn sure something’s changed about the Royal Festival Hall, but you can’t quite put your finger on it. After a £111 million refurbishment that’s been just two years in the making but several decades overdue, welcome back to Britain, RFH, and, my, don’t you look good.

For a 56-year-old, the Royal Festival Hall has had a very chequered life: unfinished in the 1950s, grossly remodelled in the 1960s, a political football in the 1970s and 1980s, and turned into a shopping mall of “revenue generators” in the 1990s era of cash-strapped arts funding.

More worryingly, its acoustics, the very purpose for which it was built, have always been the wrong side of “characterful”. Overcompensating for what its designers thought would be a too reverberant auditorium – because of its concrete shell and gaping, wide and shallow shape – acousticians filled it with absorptive materials, leaving the sound too crisp, lifeless, cold and distant. In the far corner beneath the balcony, Sir Simon Rattle famously recalled, you lost the will to live.

Allies & Morrison became house architects in 1992, after the chief executive at the time, Nicholas Snowman, bought a book on its history as a leaving present for a colleague, catching a glimpse inside of what the old girl used to look like in her youth. It was then that a path of conservative surgery was set upon – in consultation with Martin and Moro, and after their deaths, surviving assistants and collaborators, such as the furniture designer Robin Day and the architect Trevor Dannatt.

Allies & Morrison are doomed to be forever described as “polite” architects, inheritors of the pared-down modernist aesthetic that the hall’s original architects – Robert Matthew, Leslie Martin and Peter Moro – introduced to mainstream Britain, if not quite their chutzpah. But the gentle touch is just what is required when you’ve had a past as troubled as the Hall’s. “We were appointed as custodians of the building,” says Paul Appleton, of Allies & Morrison, “there to recover what it was. It was simple. Either you make the changes, or it dies.” In 1996 Allies & Morrison wrote the hall’s conservation plan, probing the building’s childhood and upbringing. And then the breath of life: a year later Tony Blair held new Labour’s victory rally there, putting the old trouper symbolically centre stage again. But it wasn’t until the arrival of the nononsense Michael Lynch as chief executive in 2002, fresh from reviving the Sydney Opera House, that the finances and the will was there for the long-awaited extreme makeover.

Allies & Morrison’s first act was to create a slim new building to the west, sandwiched between the Hungerford Bridge railway viaduct and the hall. It’s not a stunner and it just houses more cafés and shops. But at least it’s discreet, and this is one rare part of London actually starved of cappuccinos and consumerism. With the removal of the riverside road for a “Festival Terrace” (yet more bars, shops and restaurants), and the boosting of ground-level pedestrian links from Waterloo to Hungerford Bridge’s new pedestrian walkways, it finally transforms the hall’s once antiseptic environs into the “fun, fantasy and colour” that the Festival of Britain’s director general promised.

More importantly, the new building provides the baggy backstage space that the hall always lacked and offices for its staff. The building’s jaw-dropping foyer terraces and staircases, wrapped around the auditorium lifted high above your head, had become clogged with a shabby shanty town of offices, a labyrinthine souk of shops, all now flushed away so that the clarity of the hall’s original interior can be drooled over again.

Its original “main entrance” has been reintroduced, allowing the full splendour of the hall’s lavish but democratic “architectural promenade”, where social classes were free to consort, to unfold before your eyes. With its spaces, colours and tactile materials, the first truly modernist space in the city is, again, magnificent.

Allies and Morrison also removed the girdling walkways added in the 1960s and now throttling the building and returned a splash of its old glamour with the People’s Palace restaurant.

It’s the changes to the auditorium that have caused the most bother. Walk in and, again, you might not notice anything. The whole room, though, has been stripped to its skeleton and put back. Day’s seating has been beautifully reupholstered and repositioned to provide three inches more leg-room, the stage reequipped with modern lighting rigs and endlessly changeable podiums. The biggest challenge, though, has been improving the eccentric acoustics, masterminded by the world expert Larry Kirkegaard.

Its absorptive materials have been replaced or made detachable. The one major change to the fabric has been the widening of the stage area and the removal of the tumbling wooden clouds above it on the ceiling. Their replacements – by manipulable canvas sails – aren’t beautiful, but if they do add some warmth to the acoustics, it is, perhaps, a necessary sacrifice. “You have to remember the shock in 1951 of walking in,” says Robin Day, one of the surviving designers of the original project. “There wasn’t anywhere like this in London. Now it’s as it should be, there still isn’t.”

“It’s tremendously moving,” agrees Trevor Dannatt. “This building will amaze people all over again.”

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