Long arm ballcock? Wouldn't recognise one if I fell over it. The cistern is small and has a central plunger that you depress to flush. I am a great grandmother with no plumbing experience but the advice you gave sounds as if it should sort me out so I will give it a try.
Thanks for your help.
Basically .. lift off the lid .. look at the float/fill device inside.
Go to plumbers merchants and ask for a replacement assy.
Old type fill from a side inlet.
Later types (incl plastic) fill from side or bottom.
Get the same.
Usually the valve gets furred up and the washer swells.