Problems with screen pinning on HTC One M7

Recently updated to Lollipop so thought I'd take advantage of the screen pinning feature.

However, after choosing a screen to pin and it actually pinning, I can't manage to 'unpin it'. Your meant to press the home and back button together, but this doesn't work for me. It stays pinned. I've tried a lot of different combinations, lock button and back button, home button and lock button, but none of these work.

I understand that the HTC One M7 doesn't have a dedicated Recent Screens button. So is this the problem? Anyone else managing to pin screens on their HTC?