in most cables, the CPC is a size smaller than the live conductors (it is not expected to carry a current except in fault conditions; when there is a fault leading to an overcurrent or a short circuit, the fuse will cut power quite quickly).
In Twin and Earth and other flat cables, the CPC is bare copper inside the sheathing; not insulated like the red and black, but has to be sheathed when you expose it e.g. in a socket enclosure.
The sheathing on Earth wire should be green and yellow. You can buy lengths of it in hardware stores, DIY sheds and electrical suppliers.
Before G&Y was introduced, plain green was used, sometimes rather dark, perhaps this is what looks black to you.
If you have two wires twisted together, this usually happens when there are two cables (e.g. on a Ring) and the CPC from both of them has been poked into one piece of sheathing. this, and the twisting together, is no longer considered good practice.
PVC cable, unless it has been damaged by heating or contact with e.g. expanded polystyrene, or gnawed by rats, lasts a very long time, you will probably have some other reason for needing a rewire before it is past its best.
