I've just had my zip slider on the newly acquired TF break, it had completely jammed up solid, and it was getting dark; so I was rushing. I just tried to gently part the slider with a thin screwdriver and it snapped.
I actually thought I was going to have to replace the complete window, but I have managed to source a couple of sliders from MGf Mania and now awaiting their delivery. Hopefully it will be a cheap repair, but I will make sure that the zip gets at least an annual lubrication from now on.