It doesn't look like any of the standard English oral corpus, but may well be a revived broadside ballad. This contains a distinct genre of 'mad' ballads which date from the 17th and 18th centuries.

You have 2 obvious chances, either someone here recognises the plot, or go back to your informant and ask for more info such as first line or a word at least from a title.

FWIW it's usually ploughboys who go whistling 'o'er the lea' and the phrase is common in late 18th century pastoral songs that were the product of the pleasure gardens and of course quite a number of these eventually ended up in oral tradition.