The current Pentangle tour is billboarded as being a celebration of 40 years since their legendary gig at London's Royal Festival Hall. In the build up to the June 29, 1968 show, and as a masterstroke of PR savvy, Pentangle were removed from public life for three months prior, with the only views of the band being tantalising silhouette profiles on half-page press advertisements. Most daring of all, the chosen venue would be by far the largest these five folksy jazz underground darlings had ever played. Just days after the release of their eponymous first album, the show indeed marked the arrival at the top table of a band beloved of basement club cognoscenti, a duality they managed to maintain until their disbanding in 1973.

Talk of their working together again was rife last year and would have marked their 40th anniversary as a band but unfortunately, still showing signs of what separated them, a working harmony could not be found until earlier this year. As they were in their prime, these are five wholly individual musicians who can work either for or against each other.

In no small way, The Green Man Festival, nestling away in the Brecon Beacons, served as a kaftan-wearing cupid to guide the right people into the right place at the right time, and bring this band back together again. The village doctor and father of festival co-organiser Jo Bartlett was a musician - a bagpipe player - and back in the 1970s had friend in one-fifth of Pentangle, John Renbourn. John remembers Jo as a little girl running and playing on the village green where both families lived. Shooting forward in time, as curators of the best UK festival of its type with her husband and music partner Danny Hagen, both Renbourn and fellow Pentangle guitarist Bert Jansch have appeared on different stages on the same bill. In his ‘Forward’ essay within last year's The Time Has Come retrospective Pentangle box set, The Times' chief rock critic Peter Paphides infers the whole Green Man experience to be the children, both adopted and natural, of Pentangle's legacy.

So we arrive tonight in Wales' capital city, armed with both history and anticipation as Pentangle reach the orchestrally wide stage. They start with 'The Time Has Come', that reassuringly reveals Jackie McShee not to have lost command of her voice. As the wispy young icon of late '60s feminine romanticism, she unwittingly set the standard for all aspiring folksy/hippy female singers and the carefree naïve breeze of a voice still delivers. 'Light Flight' follows; a convoluting song probably better known as ‘Take Three Girls’ as it's the theme from the TV show of that name (anyone still with me?). Then 'Mirage', which possesses the soaring calm of McShee's voice underpinned with guitars, rhythmic fighting, and then resolve. Surprisingly I'm seeing in front of me a band that are hungry to make, create and move people, which is so far from what I would have expected from a group old enough to welcome this tour purely in terms of a pension boost. With each additional favourite played, these are people as in love with music as ever and thrilling themselves with instant alchemy as any fresh teenage performer can. I am truly astonished.

Behind McShee, drummer Terry Cox has his kit set up to face double-bass player Danny Thompson on the other side of the stage, so allowing eye contact and intuition to flourish between the twin foundations as before. In the intervening years, Thompson established himself as one of the most in demand session players on the circuit, so it's an extra delight to see his reputation in action tonight. At the other end of the fame scale, Cox, who with Thompson revelled pre-Pentangle in the accolade “best rhythm section in London”, jacked it all in after the band broke up to run a restaurant in the Balearics, from where they dragooned him for this tour. Getting back on the bike seems to have been all that was needed for Cox, as observing the pair of them in mutual creation it's hard to imagine him having ever retired.

Flanking the singer, bookend style, are seated guitarists Renbourn and Bert Jansch, each doyens of their particular styles. With Jansch still presenting more upfront personality and flash in his delivery, it's deceptively easy to overlook Renbourn's contribution. Only by keeping a close eye on his playing, the subtlest of mood movements and forward motion are revealed. Known for an enduring interest in sound and textures experimentation, he later sits crossed-legged on the floor to play the sitar for a couple of songs.

Loose acquaintances on the nascent London folk rock scene, Pentangle were all 20-somethings when they formed themselves into this five-pointed unit. Dotted around St David's Hall tonight are equally aged former compatriots with still-visible art school sensibilities and a hint of government-baiting idealism from their shared world-order-toppling 1968 generation. Having possibly endured the discomforts of student sit-ins, communes and less-well-catered-for outdoors festivals from 40 years back, these comfy chairs with spacious legroom seems a civilised way for band and fans to reunite. Although Pentangle play traditionally-styled folk tinged with jazz, their output is sinewy and challenging, and not wan as some might expect.

'Cruel Sister flows with exquisite beauty with 'The Snows, 'In Time' and 'The Wedding Dress' each standing out. Although the lack of 'Poison' in the set is a huge omission, experiencing these accomplished musicians as they become once more this single entity, one can only imagine how powerfully mould-breaking they were in their first manifestation.

A word of advice: surrounded by their progeny, do not miss Pentangle's headline appearance at next month's Green Man Festival. It may well be your final opportunity.