Thanks for all your helpful suggestions.
The club (whose name I won't mention) was a fairly dire place where I stopped off on my way through London. I left before the end as the audience were making more noise than the performers (why do people pay to go to a music venue just so they can chat to their mates? They can do that for nothing in any pub and without the live music to distract them!)
Back to the point. The singer was not known to the organiser as he was at the club for the first (and probably, like me, last time.) I think it must have been a self-written song.
So, whoever you are, if you read this, I don't want to steal your material, I'd just like a recording of the song, if it exists. It was about the spread of the surveillance society and the growth of the underclass and was very moving, if harrowing. I'm sorry that it was also wasted on your "audience".