Ball cock assemblies can be bought from B&Q as a complete item. Simply remove old one and replace. Nylon nuts on nylon threads are the norm unless the cistern is over about 50 years old and has never been serviced. Undo the nut with your fingers. A spanner will probably chew it up.
If it is a matter that the inlet valve won't shut you can adjust it by the nylon screw, usually found near the top where the inlet valve is located. A kitchen knife or nail file is all you need ~ a screwdriver would be a luxury.