You haven't included that debt when you set up your DMP.
Tell the collection agency that you are in a DMP and they will give you 30 days "breathing space".
In that time either speak to the DMP and see if they can add the debt on to existing plans
(may not do it, and I have seen some DMC's add "admin" charges for this with excessive amounts).
If unable to add it call the collections agency and say something like "the DMC is going to offer £X per month, but I want to get it cleared faster, Will you accept £Y per month?"
I doubt they would say no since the alternative is a DMP.
Let us know how you get on.