So when choosing a mission from the map, it gives you 2 options, to resume or to restart if you have a checkpoint active. Whenever I chose to restart the mission it always put me in at the checkpoint, so I figured it was broken, but I just found out what I was doing wrong. I noticed when I choose resume, it selects the default difficulty, then I go and choose one with heroic and select it as normal with 'A', but this time I selected the difficulty with 'X' and lo and behold, I get a message telling me progress will be reset and the mission starts from the beginning. Can't believe it took me this long to figure that out! Edit: Thanks to everyone who've been leaving appreciative and positive comments. It's that kind of thing that encourages people to share things they find with the community.