I have disconnected the inlet water pipe and wall screws, however both stud bolts with the wingnuts turn together, although I don't see how this actually attaches the cistern to the bowl. The cistern appears loose : is it a matter of pulling it up harder and harder?
the studs have a square shoulder that locates in a slot on a metal plate on the underside of the cistern,this plate is clamped in position by the syphon jamnut,try pulling wingnuts down as you try to unscrew them but maybe you've rounded off the shoulder of the stud or the slot in the plate.if you pull the cistern off with force you'll break something,try some wd40 on the threads,if that fails then you'll need to cut the studs with a junior hacksaw and replace.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix plumbing.