SOLVED (AT LEAST FOR NOW) The problem seemed to be with my actual mouse, it's coming to the end of it's life i assume as all the buttons don't register like they used to! At first I realised I could fix the problem by just spamming left and right click on the mouse until I regained control. Now, I've just switched out the mouse and the problem as disappeared! Hopefully it won't come back but it doesn't seem to be a Bungie issue, rather a hardware issue!!