Changing Stopcock in expansion overflow tank

[adrian112]

Stopcock needs changing in expansion overflow tank and not sure how to go about changing.
Overflow tank is in attic(loft) and unsure as to whether water mains needs turning off?
As ballcock connection has to be fitted to side of overflow tank and to the water flow mains, it would appear that the only way is to stop water and go from there?
Thanks you in advance

 

[seco services]

isolate yout mains water stopcock to the house