Recent experience with another ferry company.
We had a ferry cancelled for a technical issue coming home from Spain last month. We were rescheduled on the next available sailing 3 days later and had half of our fare refunded within 2 weeks. We were in our camper so we enjoyed the extra time in Spain and had our extra incidental expenses reimbursed by the ferry company too.
I would ring them up and appeal to their better nature on the price explaining the details. I'd have the actual dates you want ready in case they agree. All you can do is ask.