You're confusing that with credit agreements such as credit cards. Credit card companies do indeed sell their debt to debt purchasing company. I can assure you they buy the debt at a lot less than 50% - think single digits. I did some work for a DPC in Worthing a while back and got to see their...
As has already been said, contact Southern Water.
As the DCA has your wife's address it would mean if they, or Southern Water, did raise a claim you would get the claim papers. You then deny the debt and give the reasons why. The court would then throw out the claim. The only exception with...