A reverent hush falls on Bangor Abbey, as it must do on a weekly basis in a working church, and Kieran Gilmore, director of the splendid Open House festival thanks the assembled throng for respecting this “sacred space”. To be fair, he does not need to make the request too firmly, because the sanctuary is already silent, and the air is crackling with anticipation. It is not often that an artist as original and bold as Bonnie “Prince” Billy plays in this humble town but his delicate and strange hymnals fit the scene perfectly. “When you ask me to sing…