i was on the ferry sete nador on 10.9.09. i booked with southern ferries, no booking over the internet is possible, so ring +44 844 81 57 785. it cost 300 euros, i think the prices are more or less uniform. i think it's the same with car/moto and with or without cabin. i think the cabins are first come first serve. i booked two days ahead and got a 2 man cabin. the ferries run about every 4 days or so and mine departed at 2pm and arrived the next day at 17pm (2h time difference). make sure that whilst you are on the ship, to get your passport stamped (first) and then do the costums for your car. this was a bit chaotic, so ask around where and when thats happening.
hope this helps