Go to http://www.trasmediterranea.es and put in your requirements, dates etc. and they will give you prices and availability, or call their agents Southern Ferries 0207 491 4968/0870 499 1305. Haven't used them myself but have done plenty of bookings for others on them and never had any problems.

We've used them a couple of times but pricing is a real tangle unless you ask for a specific date and time, in our experience. The boats are excellent and if you want the experience of a short boat trip OK - very good overnight crossing.

However unless you're going for a long time the cost of taking your car is far more than flying and hiring. Have a look at Air Berlin

http://www.airberlin.com/

Prices comparable with Easyjet and Ryanair but allocate seats and manage to give you a snack and a drink in the 30 min flight. - cleaner and smarter.