Presumably you can't just nip the wing nut up a bit. If not:
1. Use
Screwfix item 517JM. Doesn't matter that it has a rubber washer rather than plastic.
2. Also replace the "doughnut" washer which fits between the cistern where the flush valve is and the bowl.
3. Be prepared to hacksaw the old wing nuts off, they look well and truly rusted on.
4. You will need to:
4.1 Turn the water off to the cistern inlet valve.
4.2 Flush the WC and sponge any remaining water out of the cistern.
4.3 If you are changing the doughnut washer you will need to undo any screws / cut through any silicone securing the cistern to the wall.
4.4 Remove the existing cistern securing bolts. Be careful you don't damage the cistern!
4.5 Remove the cistern from the bowl and clean base of cistern and top of bowl.
4.6 Replace the doughnut washer.
4.7 Replace the cistern securing bolts. If using the
Screwfix ones, the conical rubber washers go inside the cistern, with the narrow end pointing down. Tighten the bolts a bit at a time, evenly, so they don't pull the cistern out of level.
4.8. Turn the water back on and make sure nothing is leaking.
4.9 Re-fasten / re silicone the cistern to the wall.