I would write to them again, saying that you have found being in debt extremely stressful, and needed the assistance of a professional. Point out the admin charge is for the management of 7 creditors and thus minimal. Get the management company to go ahead with the payment schedule and pay the creditor the proposed payment.
Thats what I would do.
Alot of people are making themselves sick and can become totally overwhelmed dealing with irate creditors. If an intermediatory alleviates that pressure, then so be it.
Just my opinion!! Good luck.
cheers C