Unfortunately, none of this answers my question, although it has produced a few new bits of information.
I know too that it was in the repertoire of Charles Tree (1868 - 1940) along with a number of other West Country songs.
It seems unlikely that it was "first heard in the early 1800s" if Widecombe Fair started in 1850. I can certainly believe that it may have first been published in 1880. From the rest of the information from Hawker, it would appear that the death of Tom Cobley or Cobleigh predates the start of the Fair.
And if Baring-Gould collected the song from Bill Westaway's father and published it in 1890 or '95, it must have been a relatively new song at that time, assuming it postdates the start of the Fair. Interesting that Baring-Gould seems to have 'collected' a song which can hardly be called 'traditional' at the time, by the yardsticks we use these days to define 'traditional' - we don't generally consider a song written within the last forty years as 'traditional', do we? (Discuss...)
Thanks, Hawker and Jim, for taking the time to at least try to answer the query.