debt letter

You said that they'd given you a date to respond by... Is this a dunning letter, or a summons? Your best response depends on the answer to that question.

I'm assuming you've got just a letter, for now. Send them a demand for validation of the debt. There's a good letter template for that here. You're entitled to have them prove the debt.

its not a summons, theres nothing on it that says legal action. Not sure what a dunning letter is sorry.

A dunning letter is a leter that is asking you for payment of a debt.

????
A dunning letter is just a letter demanding payment of a debt. I used the term to differentiate what you got from a legal instrument such as a summons, which generally originates from a court.
Go with the debt validation letter I linked to in my previous post. Send it certified mail, return receipt requested, so you can prove that they received it. By law, they have to provide validation if requested. They also have to cease collection activity until they provide it.

Definately send them a letter demanding they validate the debt. You are looking for proof you owe what they say your wife owes. So you want to see a signed contract (or agreement), statements, etc. By law they can't continue to actively collect until they provide this information.