Replace ballcock & valve in water tank.

The ballcock and valve in my water tank are not functioning correctly and I am getting a slight leak from the tank as a result. I'd like you to replace the ballcock & valve and make sure the leak is no more - I don't know if it would be necessary/worthwhile to put a sealant where the leak is actually coming from. See pics A for the ballcock/valve, pics B for where the water is leaking and C for the result. Even when the ballcock is floating on the water there is a constant minor drip of water into the tank until it hits the overflow pipe. This is so consistent that water is dripping from the valve of the overflow pipe onto the floor below, causing a stain on the ceiling in the room underneath. I could probably replace this myself, I know it is a cheap part and not a huge job, but with water I'd prefer to have someone more experienced do it on the off chance I make a b*llocks of it and flood the room. Please be experienced in plumbing if bidding for the job!