those nice people at SeaFrance

.... have just sold me a day return to Calais for £5, with a bike. You
can't book the bike on the website, e-mail them to let them know you
are taking a bike. One thing I did notice in their T/C's is that if
they believe you are not making a day trip they can surcharge your
credit card (and you have to agree to this when you make the booking).
This is presumably in reaction to the man who bought a day return as it
was cheaper than a single and got surcharged when he didn't come back.