As noted previously 'Go Lassie Go' with the McPeakes tune was recorded in a field recording held by the British Library from 1952. It was also recorded here on a 10" HMV LP.
One reason for changing the title to 'Wild Mountain Thyme' would be to give it its own copyright identity and separate it from 'Go Lassie Go', but this didn't stop any confusion of people thinking the whole was a simple copy arrangement of a trad. work, rather than a re-writing of the lyrics with additions and a re-shaping with an original tune (or at least one which is otherwise presently unrecorded before 1952).