DNOs vary. There might be a fixed charge for the call, sometimes they are free. The work would usually be done by a company which has the contract to fit and maintain meters, as a subcontractor. I would be surprised if they charged you as much as an hour's work.
Easiest thing would be to call the number on your electricity bill and say a redundant cable needs to be removed from the meter as equipment has been removed. You are not expected to know all the companies involved in the electricity industry, and the company who sends your bill is responsible for dealing with you as their customer.
If the person on the phone is a numbskull, send a very short letter to the address on the bill, and it will be logged and passed along until it reaches someone who knows what to do.
BTW if the tail to your current CU is old and thin, it would be a good idea to get new 25mm tails fitted at about the same time. The electricity contractor will not connect to your CU though, so you or your electrician would need to coordinate the work. You could ask if they will supply and fit an isolator, though, which means the two men do not have to be on site together. Sometimes the meter engineer will call once to disconnect the old tails, then come back later in the day to re-connect the new ones that have been connected to the CU. SSE used to do that at no-charge, I don't know if they still will.
A photo of your installation will help us.
