I emailed a professor in March; no reply. I emailed again in May; no reply. I then emailed someone else to email that person to get them to reply to me.
They replied, but then took 2 weeks to reply once more, and agreed that I could let them know by X date my decision on what units to study.
I sorted it out before that date, but then get an email from the tutor that intervened to say that I mustn't leave things so late!