From that I assume the funds are going to be withdrawn from your HSBC account - in which case contact them ASAP and stop the transaction. Explain what's happend and they should be able to stop it. Paypal normally take a few days for a transaction like that to go through (so much for electronic transfers), so there time for it to be stopped. I'd also try and ring Paypal - the number is on this page
http://www.paypal.com/uk/cgi-bin/web..._contact-phone