The Green Man Festival, Glanusk Park, Powys

Alasdair Roberts is pale, dark and lugubrious-looking, with a thin Scots face perfect for casting as a religious fanatic in a historical drama. He sings and plays guitar, although the job-description of singer-songwriter - with its inevitable associations with Seventies songs about lurve for one's Laurel Canyon lady - hardly seems appropriate. Although Roberts does feature some of his own dark material, it tends to echo the hobbling metre and doomy sentiments of the old death-ballads that still form the major part of his act.

To call it an act might be stretching a point. For all his skills, Roberts is no showman, but that's one of the things that can be so refreshing about him. As far as ingratiating oneself with the audience goes, he can be as uncompromising as Calvin or John Knox, and, depending on your mood, maybe not much more fun. He can also be - as at last year's Green Man, in a dreadful, thrashy trio - his own worst enemy, as truculent and bloody-minded as the star-crossed characters in his songs. Indeed, at a Roberts performance, 100% quality assurance is never guaranteed. But sitting on a stool in a tent with only his acoustic guitar for company - the recondite tunings chiming out like bells - he completely entranced a large crowd on the festival's second stage late on Sunday night. The audience was partly converted already, with fans requesting obscure old numbers, but Roberts simply got on with it, relying on the stripped-back-to-the-bone merits of the songs themselves and his own dry humour. And what songs they are: fatalistic tales of drummer-boys at Waterloo (it ends badly, as you feared), cabin-boys at sea (ditto), and the worms eating away at his true love's heart. There's a King Jamesian intensity of language whether the words are old or fresh-minted.

As a regular performer, Roberts also in some ways personifies the virtues of the Green Man Festival itself, which this year (its fourth) moved to a wonderful new, stately-gardens site on the banks of the River Usk just west of Crickhowell. Nominally dedicated to new wave folk or what used to be known as folktronica, like The Big Chill it's another eclectic niche festival with a particular appeal for country-loving townies and couples with children, although almost anyone could find something to enjoy. Of course, the new site (with a sold-out capacity of 6000 or so) was compromised by some very wet weather, which meant that, as lazing about in the sun was not an option, the depth and breadth of Green Man's attractions were tested and occasionally found wanting. Like The Big Chill in its earlier years, the festival can appear to have too little going on for too long. If the aim is to become still larger, they probably need some bigger and better names, especially earlier in the day. 18th Day of May, who performed for a very long hour on the main outdoor stage on Sunday Afternoon, would have struggled even in a tent.

Cerys Matthews, playing the larger outdoor stage at around the same time as Alasdair Roberts, looked mercilessly exposed, with only her own semi-acoustic and a second guitarist keeping her nakedly confessional songs covered up. After that, it was a long wait until Calexico from Tucson, Arizona topped the bill with an assured performance of great power. As the sound of mariachi trumpets drifted through the lantern-lit terraced gardens; as bats flew, tired children grizzled, and grown-ups rolled another doobie, it felt like time for bed.