Only trouble with the Eisteddfod is that everyone is playing the same tunes. If the judges don't recognise a tune, you get marked down because they think it isn't Welsh.
Having said that, many of the tunes in Welsh collections are very similar to English tunes (Which came first, I can't tell). Mwmpwy (is that right?) Porteynon (Porteynon Whim) is remarkably like the Grand Hornpipe, and Galch Gwyn Morgannwg is similar to the White Joke.