How can I fix my toilet cistern?

How can I stop my toilet cistern from continually running water into the toilet pan? The float in the cistern doesn't appear to be moving high enough as when I hold it out of the water with my finger the water stops. I have tried moving the float by screwing the spindle further up and down, but it still happens. It's a continental style cistern with a flush button on the top of the cistern.