In reply to jussy: On 5th of Aug, I'm travelling to France/Spain and I've had same thing to solve.After 3-4 days of monitoring prices of various ferry companies, I decided to go for Norfolk Line as cheapest, even if not goes to Calais. I got return ticket for £60, but two days earlier (for same time, off peak, low fares) it would be £54
So, check and book ASAP, don't wait even a day if unnecessary.