your best to ring them direct - and ask if they have any advance tickets (use to be called Apex)basically these are non amedable non refundable tickets so you most travel on the time and date stated on the tickets - but they can save you buckets.
sorry havn't got the number to hand it's packed away ready to move.
Also try P&O who are doing a summer deal. We got a crossing for 4 and a car return for £96. I compared with Eurotunnel which was well over £200, Also tried Speed Ferries but they were fully booked on the days we wanted, I think they get booked up very quick as they are so cheap.