I was in similar situation with you, received two "solicitors" letters on last June and July, and two debt warning letters earlier. L have sent a reply letter on 09.07.2013 including action fraud reference as suggested by CitizenB. Thanks to him and everyone here, I have not received any more letter from them.
Of course I am not using my Paypal account for the last 5-6 months and I will never use it again.
Best wishes..
Selsipahi