You can still open a dispute even if you made a guest payment but you need to contact customer services and they will do it for you. Click on the words "Contact us" at the bottom of paypal pages > click on the "call us" option on the left (fees "may" apply so check the cost with your phone tariff first, as you sometimes have to wait a while to connect). When you get through don't select any options just hang on till you get transfered to an agent. OR have you considered contacting Customer Service via Facebook or Twitter? It's: https://www.facebook.com/PayPalUK and @AskPayPal for Twitter.
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