I believe it's a mystery, and I've read about that Welsh suggestion before. Another thing is that I always write it as a single word, unhyphenated. At one time the Ordnance Survey got into hot water for adding a superfluous "hill" after the name.

Nearby there's another favourite of mine that's a "Pen-," namely Pendle. I believe that it's incorrect to refer to it as Pendle Hill.

Anyway, they're both lovely and they're both in God's own country!

What group behaviour, Keith? We're all rugged individuals here!