There are good debt management companies around and there are some very bad ones who expect to take “up front” high fees or sharp practices, which get everyone a bad name, it is difficult to know which are which and personal recommendation is important. I assume you had a debt management agreement with your creditors through a debt management company who were charging large fees and you have now come to an agreement directly with your creditors thus saving the fees, but one creditor is being difficult and taking your payment as interest, therefore the capital sum remains the same and may even be growing. This is the problem with informal agreements they are not legally binding.
Have you considered an individual voluntary arrangement (IVA)? This has to be dealt with through an insolvency practitioner (IP) and is often what companies are talking about when they advertise saying such things as “write most of your debts off in 5 years or the government will reduce your debts etc”.
The advantage of an IVA is that if you own a property or do not wish to enter bankruptcy it can sometimes be the answer to your debt problems as it can write off a considerable amount of your debt without putting your property at risk. The IP draws up a proposal for the creditors on a pro-rata basis, and if creditors holding 75% of the amount of debt agree the other 25% are included. The IVA does not usually provide for payment of the full amount of debts and are usually for no longer than 5 years, once agreed the agreement is legally binding. An IVA may not be right for everyone but it certainly has its place in the right conditions, your county court may give you a list of local insolvency practioners, it may be worth talking to some of them