Old, old argument. Would be interesting to find some new information.
Always reckoned to be a photo-finish (or photo-start) between 'The Topic' in Bradford and 'The Ballads and Blues in London'. MacColl was always sure that it was B&B, but there is no documentary evidence.
There was a 'club' at The Theatre Royal, Stratford East (Theatre Workshop) prior to the Ballads and Blues, but I get the impression that these were concerts rather than a club. Perhaps the forthcoming Ben Harker 'MacColl' biography will give us a clue.
Jim Carroll