I've just worked out how to do this. Select Settings from the drop-down list (top-right in the purple banner- your email address) then Mail from the drop-down and tick 'Empty trash on logout'. Then logout and log back in and all the trash will be gone.
An easier way to achieve emptying the trash is to hover over the Trash folder in the left hand column below your Inbox and you will see that a plus sign and a trash bin icon is revealed. If you if you hover over the trash bin icon you will see it reveals a message "Empty Trash Folder" click on the trash bin icon it will empty the trash contents without the need for you to log out and then log back into your email account.