Numerous claims - among the the earliest were the concerts at The Theatre Royal, Stratford with MacColl, Lomax and others which later became The Ballads and Blues (MacColl again, Isla Cameron, Fitzroy Coleman and occasionally Joe Heaney Bert Lloyd and Seamus Ennis (among others)
Have poor quality snippet of a radio programme somewhere on this.
The Topic Folk Club (Bradford?) is also an early claimant.
Jim Carroll