I would advise anyone in this predicament to phone the utility company concerned and ask to speak of to the head of the book-keeping dept. It is no use speaking to whoever the phone called is just passed through to. You need to explain that you are not the debtor concerned and give all details about when you moved in, who the previous owner of your flat was and their solicitor's name and address.
If this does not solve the problem, you need your solicitor to write to the company involved.
If your own payments are up to date, or if you are using a different utility company, the company who are owed money cannot just disconnect you.
Once the matter is sorted out, and if the new owner is using the former owner's company, I would move to another utility company.
There is precious little a debt collector can do, as you can prove the debt was incurred by someone else, before you bought the property. However, they can be annoying to get rid of, so try the other route first.