Have you tried looking at them via a web based Browser. click here Go to the ADVANCED log in put in your pop.tiscali.co.uk into the first box, your full e-mail address in the second and your password in the third.
See if you can view the e-mails from there. It may be they are too large or corrupted but at least you may be able to identify who sent them. From here you can also delete them if necessary.