Too late for Paypal etc now. I'd be suggesting you lodge an enquiry through the Post Office, I think it is best for the seller to do this although the addressee might be able to do so too. Here is some information I found on UK's Royal Mail website about the process and compensation etc (note Surface Mail is now called International Economy)

http://www.royalmail.com/personal/help-and-support/royal-mails-claims-process-for-loss-damage-and-de...

http://www.royalmail.com/personal/help-and-support/available-compensation-for-lost-or-damaged-intern...

http://www.royalmail.com/personal/help-and-support/claims-process-for-international-items