1) The debts are subject to a charge are allowed to charge interest only set by the Court. If the Court did not allow interest then it cant be charged
2) Your credit score only goes down if there is a valid CCJ
3) Yes you can pay by installment
4) No you can't have conditions attached, you either owe the amount or you dont
5) You can agree to a charge but they can;t enforce whilst you are making agreed repayments
Does that clarify?
You need to write and make an agreement, it is as simple as that
You need to make payments as per the charging order, unless you apply to vary that.
The £3400 debt then you need to come to an arrangement.
Does that help?
When you attend Court or you can ask for it to be varied later.