A mercifully modest makeover (thank the skateboarders) of these brutalist beauties revives something of their 60s soul, though certain details lack a guiding hand

“If goats could climb over buildings, why couldn’t people?” says Dennis Crompton, veteran of a cell of young tearaways who, ensconced in the architects’ department of the London County Council at the dawn of the 1960s, designed the Queen Elizabeth Hall, Purcell Room, Hayward Gallery and their connecting tissue of walkways on the South Bank in London. Crompton and his colleagues wanted the project to be like theMappin Terraces at London Zoo, of 1913-14, where captive beasts roam over a concrete mountain range, only for humans. Now renovated, a half-century after its completion, the Thameside complex is inching towards being the place that Crompton and his comrades had wanted.

The craggy, layered geology of theSouthbank Centre was conceived as a series of platforms, inside and out, where people might do what they will, unified and solidified by flowing, mouldable concrete. A collage done in 1972 portrayed the architects’ wishes for the by then completed complex. The work of another of the tearaways, Ron Herron, it showed bright colours irradiating the grey video screens, a pink tent, clip-on structures, artificial turf on the decks.

The way Crompton tells it now, touring the newly restored buildings, their attitude was generally laid-back. Interventions by artistic grandees such as Yehudi Menuhin and Henry Moore had to be accommodated. Some guile was needed to smuggle the radical design into being, under the uncomprehending nose of the head of the department, Hubert Bennett. “We were fairly relaxed about the whole thing,” says Crompton. At the same time, the work has craft and form. Platoons of draftsmen were required to draw the layout of the boarded formwork that left its patterns on the concrete, and platoons of carpenters were required to build it. The essence of the project is its interaction between the fixed and the mobile, artistry and action, permanent and temporary.

Later, something got lost in translation. The terraces came to be seen as inhospitable and obdurate, not least by successive managements of the South Bank. Since the 1980s, a procession of eminent architects, collectively clocking up God knows how much in fees, have produced one grand and ultimately foundering scheme after another, all of them working from the assumption that the place was a terrible failure, in need of livening up, and that the best agent of such livening would be dollops of retail, food and beverage outlets, which conveniently would help pay for the grand designs.

Now, finally, the Hayward and the building that contains both the Queen Elizabeth Hall and the Purcell Room have been renovated – the gallery having reopened in January, the concert halls will follow suit on 9 April – and it turns out that the best thing to do was not very much at all. The architects are Feilden Clegg Bradley Studios, who were first appointed to deliver an overblown, competition-winning scheme of their own. This having gone to the same graveyard as several others, FCBS have instead done a bit of polishing, cleaning and tidying up, and some ad hoc insertion of such things as extra dressing rooms and other back-of-house facilities into the spare, odd, non-specific spaces that the original freeform design generated.

So the main venues glow with renewed lustre. You can see again what a terrific room is the auditorium of the Queen Elizabeth Hall, with its poised, tuned composition of rough and fine, its sculpted concrete, luxuriant leather seats and shimmering rhythms of timber. Good acoustics, too, say better judges than me. The Hayward Gallery’s exhibition spaces, as previously reported, hum. “It’s amazing, though I say it myself,” says Crompton, of the Hayward’s virtuoso segue of staircases, pillars, beams, ducts and lift shafts, formed out of an orchestrated lava flow of concrete.

For this outcome we have to thank the revolt of the skateboarders who long colonised the mushroom-columned undercroft of the complex. They may have been precisely the spontaneous, creative users that Crompton and co had hoped for – cave-dwelling goats, if you will – but their den had happened to become prime real estate. The Southbank Centre wanted to relocate them and put money-spinning restaurants there instead. Protesting, and supported by the then-mayor Boris Johnson, the skateboarders prevailed. With the loss of this keystone, the centre’s ambitious business plan collapsed; it had to go for the low-cost option we have now. (Relatively low-cost that is – it still required £35m to make over the two buildings.)

The reduced budget isn’t entirely a blessing. There’s a bathetic splodge of blue carpet, with cheap white nosings, between that fine auditorium and the Queen Elizabeth Hall foyer, whose riotous array of triangular roof lights, scalene and inverted, has happily been restored. The more peripheral spaces still feel a bit weary and uncared for. There’s a just-about-managing quality to some of the details and finishes. Some of the cash blown on fees over the years would have come in handy here.

There’s also a lack that is to do with attitude more than money, which is of a guiding intelligence to the renovations and adaptations. The foyer bar of the QEH has been provided by another firm of architects, with jarring and tacky furnishings in gold, blue and red. The signs over the front door use the Southbank Centre’s new corporate identity, which doesn’t have much to do with the spirit of the place. Under the outgoing artistic director of the centre, Jude Kelly, its platforms have filled with artistic and commercial happenings – street markets, restaurants in shipping containers, fountains. Although in the spirit of the original intentions, much has been carried out crudely, as if just plonking them down was all that was needed.

The trick that the centre’s management still hasn’t pulled off is this: to direct with grace the smaller decisions such that they contribute to the whole, without stifling spontaneity. The centre still hopes to build a glazed foyer linking the galleries to the music venues, which I profoundly hope never happens. “The whole way of operating as an artist,” says Crompton, “is through wit and humour and insight.” This needs to be applied not to more construction, but to the curating of and caring for this extraordinary place.