Successful PPI Claim but what debt to pay first?

If you already have the cheque or it's been paid into your account, i would move it asap. You are allowed to keep something [given to you if they haven't checked your previous deb t status properly] if you think you are rightfully receiving it and as you've been told the debt has been sold on, there's no reason for you to think otherwise.
If it were a bank or building society writing to you, I wouldn't do that, but a catalogue will be more likely to have sold your debt with no clawback than a bank.