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I did get the phone number from the internet, and before I managed to ring them I also got another email from paypal to say that a limitation had been placed on my account because of the recent transaction.
This was confirmed by the guy I spoke to, and the limitaton still applies.
I have since had another email stating that the my claim was genuine and the money will be returned to my bank account.
So it all looks very genuine, but at the time was a bit scary, especially as the request for my card details came up even when I logged in via their correct website.
I can only imagine that the reason for picking me for a "random" security check was because of the sudden large transaction on my otherwise seldom used account.
So, lets hope all is OK. As I said, AVG, Spybot and Malaware found nothing......so fingers crossed.