I used to sing kiss o. If The Blessed RT wrote thistle, then fine but words of songs being altered slightly through mishearing, misremembering or thinking an alternative works is all part of what we call the folk process.

In his song Valerie, I always sung "She's got gonorrhoea" so I felt rather embarrassed when I bought his Acoustic Classics album earlier this year, to find, now I hear it clearly, "She's got gold in her ear."

Whoops...