Get some advice from an insolvency firm as perhaps there are other options which would give you legal protection. I do agree that you should speak to your DMP company and hopefully they can put a stop to this.
Hi Callum - I'm sorry to say that a DMP is not legally binding like an IVA is so creditors are free to change their minds, add interest, chase the debt, send in the bailiffs etc. It would not seem morally fair to do so if they have agreed to the DMP but they are within their rights to pursue the debt. Speak to your DMP provider to see if they can negotiate for you.