This from their website
Eurotunnel Ticket Terms and Conditions
Failure to complete both the outward and return journeys in respect of a return booking will invalidate your booking and in the event that you complete only one journey in respect of a return booking, you will be liable to pay the difference between the price that you paid for your return booking and the single fare applicable at the time that your journey was made. Eurotunnel reserves the right to obtain from you payment in full for all sums so arising. All travel must be completed within 1 calendar year of the date of original purchase.