NRAM - PPI success now DCA want money

I am enquiring on behalf of my husband. I sorted out the PPI and NRAM paid out - all through the help of MSE! Massive thanks!
However, strangely enough, as soon as the payout was, Mortimer Clark solicitors or Cabot, have requested money!
Background - husband made arrangements to pay via NRAM, then they sold the debt to Cabot, who then sent it onto Marlin. Payments stopped for about 12 months as there wasn't any communication from DCA. Husband had the top PPI on the loan so I suppose they weren't bothered at the time as they thought they would get their money one way or another.
I did a CCA request about 12 months ago to the DCA. They sent a letter back saying as they weren't in charge of the loan, that they may not have the paperwork but would forward it to him if they got it. Anyway, 12 months passed, and nothing. Great we thought. Until - claimed the PPI and surprise surprise, as soon as they payout came through, Mortimer Clark were back in touch with a copy of the front page of the loan application, but no terms and conditions on the back.
What I am wondering is that as they took so long to reply with the CCA request, is the agreement unenforceable? Is there a letter I can send to them to basically say something along the lines of "oh its convenient now to answer our request as you think we will pay off the loan with the PPI money - well do one!"
My husband is leaving his job in February and going self employed so we need the money for that, and as a back up to pay bills (I can't work due to ill health).
Sorry for the long story. Hope someone can help me please. They want an income & expenditure form back very shortly and I don't want to go down that road again.