Quick repair for cracked wallpapered ceiling
Ok so in the end I cut away a small section of paper where the crack was to 'level' out the surface. I then just filled in the gap using polyfilla, sanded and painted - yes it doesn't look very good but it's a massive improvement on what was there.

I think the correct fix would be to cut away a larger section, use special tape to repair the crack and then paste new paper over the area - using a non uniform cut (zigzag/rough). At least that's what my old man told me!

Ultimately the best thing to do is replaster it.