Cold water header tank overflowing?

I noticed recently that I have a float valve in my coldwater header tank that is not completely shutting off and passing a small amount of water. Eventually during periods when there is no hot /cold water usage (overnight) I have water passing though the overflow.
The stopcock is in my garden but has not been used for donkey's years. I am loathe try and operate it for fear of it shearing. My question is could I get a decent shutoff close to the header tank using a pipe freeze kit.This is a 15 mm vertical copper pipe. Any recommendations welcomed.

Thanks