£300 will get you one guaranteed - I've used this seller before, and although encountered a problem, not only did they sort the problem quickly [they upgraded me from a 300d to a 350D free of charge, they even gave me some freebies too!!!], I would definitely recommend them.
http://cgi.ebay.co.uk/CANON-REBEL-XTi-EOS-400D-CAMERA-LENS-4GB-CF-6-BONUS_W0QQitemZ180276502984QQihZ008QQcategoryZ31388QQtcZphotoQQcmdZViewItemQQ_trksidZp1742.m153.l1262
A little more than the Canon one [if you win], is also refurbished by Canon, but they throw in some extras which would add up if bought seperately....
You could even get one of these too, and still be under your £500 budget
http://cgi.ebay.co.uk/NEW-CANON-EF-75-300mm-F-4-0-5-6-III-USM-LENS-in-UK_W0QQitemZ200244679910QQihZ010QQcategoryZ30070QQtcZphotoQQcmdZViewItem