Question about the EPQ

When you start sixth form, you already have a set timetable. If you sign up to do the EPQ and haven't already said you want to do it when you enrolled , will your lessons for the EPQ be scheduled around your current timetable or does everyone who chose to do it have to go to timetabled lessons which could lead to a clash with your current timetable?
If so, what would happen if the EPQ lessons clash with some of your other timetabled lessons such as maths?