"Heard him do it an a club in early 70's where he said that "5 or 6 sets have appeared recently..." but as none seems to be found anywhere ..."

On the CD 'The Muckle Sangs: Classic Scots Ballads', CDTRAX 9005, Greentrax Records, 1992, there are two fragments of 'Tam Lin' recorded from the Scots Travellers Betsy Johnston and Willie Whyte. The notes to this CD seems to consist of an extended essay, by Hamish Henderson, on Scots balladry and there is not much information about the actual recordings. I think, though, that the two fragments were recorded in the 1960s. I seem to hear elements of both tunes in Bert Lloyd's tune (but don't take that as gospel because I'm not good on tunes).

Bert seems to have been a 'sly old fox' and I suspect that there was some basis in fact and actuality in everything he did - he just tended not to elaborate on his sources, that's all.