Go to http://webmail.virgin.net
. When you first do this you will probably get directed through their 'we're updating our service' screen first ... just bear with it. Give it your email address and after a pause it should move you on to the login screen.
Once you have got to the login screen, and logged in with your virgin.net email address and password, you will see your webmail inbox. Look on the left of the screen at the bottom and click 'mail' under the settings heading. In the settings screen, you have a choice: either click the Setup tab and then fill in an email address to auto-forward everything to, or else click the Vacation tab and input a message telling people you're not using the address any more and giving them the new one.