No admin fee if you do it 6pm the day before the ticket has been booked for is how I read it. It is spelt out more clearly on the
Refund Claim Form:
Also under our new “Book with Confidence” policy, we appreciate that from time to time your plans may change and you may not necessarily know when your new travel dates are. In this case, as long as you notify us before 18:00 on the day before travel we will allow you to cancel your journey and to receive an eVoucher (eRTV) for the full value of your ticket, which will be valid for use for 12 months.
Within the form is an option for "Cancel Journey - Exchange for eVoucher". The process though seems needlessly complicated for a ticket issued online.