Overflow from header tank dripping

The external overflow from the loft header tank above our combination bolier is constantly dripping (sometimes more). I've checked the floating ball valve, and it needs replacing. I've ordered a replacement, but am not sure whether just turning off the mains water and switching off the boiler is enough to isolate the header tank while I change the valve.

Is there anything else I need to do to ensure the change goes smoothly?

mains water will be enough.

Great, thanks a lot. Should be straightforward then.