Overflow problem

[IMW7659]

Before i start bear with mecos i,m not that good at plumbing!!!!Ive had an overflow leak for a while, it used to drip a little bit but gradually it has gone worse.If i run the taps in the bath it stops the overflow-for a short while.
When i went into the loft there is 2 "tanks" one bigger than the other.The bigger ones water level is above the overflow pipe,so i,m presuming thats the prob?The smaller one is below the overflow pipe,so thats ok?
Do i replace the ballcock or what?If so do i need to turn the water off,hot and cold?And where would the stopcocks be?
If anybody replies to this please put it in the most simplest terms poss thanks!By the way the house is 12years old dont know what boiler ive got if that is any help at all?Thanks

 

[ollski]

Yes its the bigger one , thats why it stops after you use hot water because you have drained the level in the tank. Isolate your water at the mains and undo the hex nut on the ballcock inside the tank and screw the new one on, this will leave you with a shiny new bit that you can throw away. This way you should not be disturbing anything that can leak outside the tank.

 

[IMW7659]

Cheers mate-sorted!