Thank you for the suggestion that it could be a date. This seems plausible, given the position of the words.
Hugh was the son of Lewis Nanney and Jane Hughes, born in Talyllyn c1635, and so had moved away from home.
I will visit the church again in the summer, to see if the inscription is still legible. I didn't notice it on previous visits and, because it is on a cliff overlooking the sea, suspect that the weather has now rendered it illegible.
Many thanks for your help,
David