I'm not sure that it would be possible to find the "original" words. As with almost all folk ballads, there are many versions, some of which vary widely, but follow a pattern that indicates that they had a common source, sometimes centuries back. Finding the original pre-"folk processed" ballad is sometimes simply impossible. Or often folklorists will argue over which of a couple of older versions may the "original."

I heard a couple of different versions of this ballad even before Simon and Garfunkle did their rendition of it, and I've heard still different versions of it since—but all obviously the same ballad, The Elfin Knight, Child #2.

Here's a version of it, complete with some pretty good information, on Contemplator.

And Wikipedia has a pretty good article on it: Scarborough Fair.

With all of the different versions, it would be hard to say whether or not Robert Westall took an existing variant or wrote his own version of it. But it's close enough to be identified as Child #2.

I hope this helps.

Don Firth