I would open the letter because a court is not a DCA.
I would bill the guilty party, Nationwide or KDP, and start adding up what they owe you ... I guess it will be Nationwide given how quickly resolvecall backed off before?
Ceylon said "you would not need to go to court simply put down what you have done and how they have failed to supply you with anything and have broken the estoppel"
You respond to the court telling them that you never refused to pay the debt if they could show it existed in the first place. You asked for verification etc. of the debt. You gave them an adequate length of time to respond. They did not supply the 'reasonably requested items'. You got them into a tacit agreement that they had no claim upon you and now, AFTER 30 DAYS, 'they are in an estoppel by acquiescence'. Also remember that you are your person's authorised representative so write it like that.
They have broken these agreements knowing that they would incur fees. You never know; you might end up with a tidy sum of money if they risk pursuing this matter in court.
OBVIOUSLY ALL THIS STANDS TO BE CORRECTED but that's what I'd do ... I take great encouragement by how quickly resolvecall backed off and wrote a letter with not a trace of a MR on on it because that seems to confirm that the letters have legal force.