I did this for a friend last week as both cables had dragged on the road but luckily for him it was near the plugs(he was using the Y extension to the 13 pin socket)
On my van I have fitted curly cables by retaining the existing 7 pin plugs and replacing the fixed socket for the curly cables with trailing sockets which are connected under the A-frame cowling.
You could make a repair in the same way.
Cut off the black cable where its damaged and fit the 12N plug to the shortened end.
If you do one wire at a time in the plug you will not get them mixed up !!
You can then fit a curly cable with trailing socket to plug into.
The curlies come with a 12N/S socket like you have on the back of the car to fix to the A-frame cowling and a curly lead with a 12N/S plug on each end.
You could either fit them as supplied or replace one plug with a trailing socket to plug your existing lead into.I favour this method as the sockets fixed to the cowling always crack the plastic.
You can do all the above cheaper with a length of black 7 core cable with a 12N plug on one end and a 12 N trailing socket on the other to plug your shortened lead into.You can remove this when parked as an anti thief measure as well!
Sorry to disagree with Paul but Scotchlocks are usually used on the car rather than the caravan and I would avoid using them at all costs as the area of contact is very small and they usually give trouble,especially if moisture gets in.
Hope that helps.