The real problem after such a long time is that your selling records will have disappeared unless you are using the seller hub which has records for 90 days.
If they go the Paypal route you will have to refund in full, and will then have to attempt a claim on the Courier or Royal Mail which ever you used.
You are likely to find the only details you have so that you can complete a claim form will be the Paypal records.
If you had tracking you should have added it when you marked as Dispatched.
