We need to get the toilet tank off and be able to put it back on. The wing nut on the left seems to be threaded. There seems to be no access inside the tank to hold the top of the bolt. If we hack saw it off then how can we re-attach it later?

The wing nut on the left seems to be threaded.Are we talking about the toilet supply nut or a tank bolt? Back to m you. Tom

Tank nut - the whole bolt just truns with the nut. I have tried holding theb bolt with pliers but no joy.

Regular tank bolts have screwdriver slots and don't use wing nuts. (see image) hack saw off the bolt and replace with regular tank bolts. Good luck, Tom

The tank bolt should have a washer under it. Try grasping it with a pair of vice grips.
Does the tank have any play in it? IF so, try cutting the bolt between the tank and the bowl. Wrap one end of a hack saw blade with tape to make a handle. Slip the blade between the tank and the bowl. If you have any play in the tank you may be able to carefully slip a blade screwdriver between the tank and the bowl to keep them apart as you cut.