Doesn't this all boil down to one simple question of motive and context?

Racist words in a song don't necessarily make the song racist - ask Randy Newman (Sail Away, Rednecks) or Jake Thackray (One Of Them). Their intentions were to reveal and ridicule racism.

Hugh of Lincoln contains no racist words but it is a racist song, as its intention is to promote racism.

So if we're sitting in a room with someone who sings Hugh of Lincoln, we want to know his intention: is the singer a racist, because the song certainly is? The way the singer introduces the song gives it its meaning in that context. When the singer sings the song without comment, then we are entitled to ask questions of motivation and judgement.

I, for one, am uncomfortable being sung to by someone whose motives may be dishonourable. I feel that remaining in that audience gives my tacit acceptance of the unacceptable. From the info our originator, Paul Seligman, has given us, it seems Alasdair Roberts is not singing the song for racist reasons. But he doesn't help himself, or us, when he sings the song without giving any introduction or context, as he did on both ocassions.