I haven’t played the Magnificat but I’ve played some Christmas music by Hayes. If it’s like the Christmas music, it won’t be difficult. Somewhat pop oriented. If I remember right, the harp part is basically written in one line. It’s more the upper register, “sparkly” notes for effect. I play lever and there were large breaks between sections where the harp played so plenty of time for key changes. There was also a rehearsal recording available which really help as generally I only attend the dress rehearsal and the one previous if I’m available.
Tell us what the Magnificat is like if you play it!