I know this is a bit late in the day to answer this question but this may be relevant to others.
A creditor will only freeze the interest if they think you have no chance in paying back the debt and if you are close to bankruptcy. Credit companies are businesses where their sole aim is to make a profit, they'll try and get as much as possible out of you if they can. Remember if you go bankrupt they'll get a really poor return, so freezing interest is a ploy designed to delay your bankruptcy.
Debt management should really only be a short term strategy which should not last any more than 5 years. Anything longer than that and you should consider an alternative like an IVA or PTD (Scotland) or even bankruptcy.
Debt management plans are relatively easy to do yourself. All that is involved is basically writing letters to your creditors saying this is what I need to survive and here's your share of what's left. If you are interested in doing your own debt management plan then visit here
http://www.debtwatchd...-Debt-Management.html for letter templates, guidance and support (via the forum).