There's a version of True Thomas told by the great traveller storyteller Duncan Williamson which brings together various strands - it's included in "The Thorn in the King's Foot, Stories of the Scottish Travelling People", by Duncan and Linda Williamson, published 1987 by Penguin.Out of print, but there are copies in libraries.

Here is what Hamish Henderson wrote in his introduction to the book (this is included in an anthology of Hamish Henderson's writings). I think it's relevant here, because it throws a light on what it means to say a version and a variant is authentic:

Both these ballads (Tam Lin and True Thomas)are now very rarer, and both (in Duncan's narrations) are wonderful examples of an "internal" collaboration of re-collection and re-forming in the mind and on the lips of the singer; together with an "external" searching for the missing parts of the narrative he did not hear until in later life, in his mid-fifties, from older traveller relations. Duncan's balladry is re-creative folk art in a quite astonishing individual manifestation. We are privileged to be in on the same process that in earlier centuries must have produced many now standard versions of the great classical ballads.

I haven't got the book with his True Thomas here, I'm afraid - but here is another story by Duncan Williamson, from a site designed to provide material for "teachers wishing to incorporate aspects of Traveller culture into the Literacy Hour" - The Traveller woman who looked back"