Hi there Richard
Shocksoc is right that the unsecured part of your Northern Rock debt should ideally be included in a debt management plan as all of your unsecured creditors should be treated in the same way. However, at the end of the day a DMP is informal meaning that there are no legal rules and the plan has to work in the best way possible for you. If you break your agreement with Northern Rock, you are going to lose the benefit of the favourable rate of interest they are currently gving you. Given what you believe you can pay back, with the extra interest and charges you incure, you may then find that your debts begin to rise faster than you are paying them off. It may therefore be prudent to continue paying NR as normal.
To answer your other questions:
1, Will the c/c companies freeze the interest, or just reduce it?
No one can predict with any certainty what the creditors will do. With any luck once they see you are paying as much as you can on a regular basis, they will reduce their charges. Some may even freeze them. However, it would be wrong to suggest that there are any guarantees of this or when it might happen as thre are no guarantees.
2, are any of my personal possessions under threat (ie will ballifs come knocking)?
No, unless you have anything of particular value. Given you are maintaining reasonable payments each month, your creditors should reduce their action against you.
3, Will the debt ever be written off? or will there come a time when they might consider taking partial payment in "full and final"?
No, your debt will never be written off unless you use a more formal slution such as an IVA or bankruptcy. Having said this and as Shocksoc has pointed out, there may very well be opportunities to offer full and final settlements in the future if you can get your hands on a lump sum.
The two main free debt management companies are PayPlan and the CCCS. It would be worth your while giving them a call. However, even with the CCCS charitable status, even they can not guarantee that interest will be frozen.
Good luck with it and let us know how you get on.