"Lady Isabella" was on "Hampshire - A Musical Portrait", an LP by various artists released in 1978 by Forest Tracks. It's not traditional (though I've been singing it for 25 years) - I think the writer (and singer on the album) was Graham Penney, who has written a number of very good songs (The Channels, The Knife-and-Scissor Man, Ladies of the Night, etc). Forest Tracks are believed to have plans to rerelease the album on CD.
I suspect the reason "Isabella" was used in the lyric in preference to "Maybelle" was simply a matter of scanning.