Following the advice of @Gordon Shumway I have searched for and found a public domain PDF of volume five of Cecil Sharp's "Folk Songs of England", which features top line and piano for 15 Sussex folk songs collected by W. P. Merrick; the first 14 songs have accompaniments written by RVW. Here is a link to the collection of all five volumes: https://imslp.org/wiki/Folk-So.....%2C_Cecil)

[edit] - These volumes are not public domain in EU, and likely not in US. The collection is hosted in Canada, where the copyright extends to only life +50 years. 

This represents a good starting point for my extended search, and so thank you all for your advice.

With best wishes,

Peter