Help please with central heating issue!

[Speeed Freak:)]

Any plumbers about?

The small tank in the loft ( header tank?) keeps over flowing, constant drip!

I've changed the ball cock but it still does it, the valve attached to the ball cock seems to function ok. If I empty some water from the tank within a day it's full and over flowing again.

Bleed all of the rads all ok

Any one help?

Thanks
Andy