You are able to resend your ticket to yourself by going on to https://www.nationalexpress.com/en and clicking on "Manage my booking".
Once there, you will enter the confirmation reference number. The next screen will ask you for the email address used to purchase the ticket.
Your ticket details will populate and you can view your ticket by going to the "View my ticket" tab and clicking on "Continue".
There, your ticket will be shown, you can print or download your ticket on your mobile device.