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.
steeveepee, December 2008