Hi there,
I'm about to be in a similar position to yourself - after getting nowhere with explaining our circumstances with the lenders and having to get to a position where we simply cannot afford to make all the non-priority debt payments.
First things first, have you sent a CCA request? I'm definitely no expert but its worth checking whether the debt is enforceable or not (even though its recent you never know) - also, did you receive a default notice before this letter from the debt collection agency?