There is an elephant in this very large room and Seasick Steve has decided to address it. It’s a question of authenticity and, not to put too fine a point on it, whether his post-fame image has been substantially built on a big lie.

It’s 10 years since the man then saying he was born Steve Wold first lurched into view on Jools Holland’s Hootenanny, bashing wildly at a three-string guitar and spinning yarns of a life spent busking, sleeping rough and jumping freight trains through the Mississippi delta. It made him the ultimate poster boy for blues authenticity: the hobo turned musical hero.

Except that a few weeks ago, an unauthorised biography blew some sizeable holes in Seasick Steve’s very marketable backstory. It found that his real name is not Wold, but Leach; he is 10 years younger than his claimed age of 75; and, most damningly, he spent the 1970s not drifting and begging but as a session musician, playing in funk and disco groups and singing backing vocals on a Mike Love side-project. These revelations arguably leave Seasick Steve marginally less credible as a Delta bluesman than Hugh Laurie.

It’s a damning charge sheet, and one that he determines to answer. “There’s been so much stuff written about me in the press, and I guess four or five of you may even care about it,” he tells the largest UK gig he’s ever played. “Well, when I wrote a song about leaving home before I was 14, and a song about roaming around jumping trains as a stupid teenager, and a song about being locked up – they’re because I did all those things.”

Watch Channel 4 documentary on Seasick Steve

It’s a carefully worded justification, of course, and one that nods to the most likely reality – that Seasick Steve is neither a lifelong drifter nor a shameless charlatan, but an artist who cleverly buffed up a handful of true early-life hard-luck stories to make them his USP. It is his huge good fortune that, going by the roar that Wembley offers up, it appears that his fans could not give a damn.

This is primarily because, chancer or not, Seasick Steve remains a fantastically compelling and charismatic live performer. Alone on stage except for a drummer for the vast majority of his two-hour set, his surprisingly nimble fingers coax or hammer extraordinary noises from his selection of battered guitars, while his voice is a rich, weathered growl. It would be a hard critic indeed who argued that he doesn’t have the blues.

There is no doubt that his credibility has taken a hit, and you raise a quizzical eyebrow at new songs such as Gypsy Blood that detail his supposed former life on the road. Yet Seasick Steve’s hypnotically rudimentary rock’n’roll retains its primeval power and the closing protean thrash of Dog House Boogie finds him still in character and drawling about “bumblin’ around and livin’ kinda hand-to-mouth”. After all these years, it’s his tall story and he’s sticking to it.