My cold water tank in the loft doesn't seem to fill up with water. It looks like the ballcock is covered in limescale & hanging down. The top feels damp and when you move it nothing happens. So i assume i need to replace the whole thing? Is there a universal size for a water tank? Are they easy to replace?
Also, i understand i will need to turn the water off, do i do that at the main stop tap? And if so, how do i manage the water inbetween the stop tap & the cold tank?

I'm grateful of any help
Sorry about all the questions?

You only need to replace the ball valve, which is the valve, arm and float. Valve and arm are fixed together and is about 贈6-贈8 from somewhere like B&Q or good plumbers merchants, and you need to replace the float which is about 贈2. Turn water off at main stop cock and the water won't move as long as the stop cock is fully turned off.

Turn stop cock off. Drain a bit of water out of tank by opening taps. Get new ball cock with valve, make sure it is for high pressure, low pressure are for toilets that are fed from water tank. Before you fit it look at it properly to understand how it works and how to adjust it. Fitting it is a reversal of removing old one. You will need two adjustable spanners for this. When you get ball cock get jointing compound, smear a bit round the mating faces before putting together. Do nuts up tight. Put water back on at turn off only a little and then go and adjust ball cock until water is at right level.
You will notice that there is a pipe to take away any water if you let it get to high. You have to set water level at below this outlet. Good luck, if you are not sure flash those lashes at some guy to help you. Sure you will manage though