Gill,
I'm after booking a ferry for the same crossing next year, i think Brittany are the only ones who do it, so i went onto their web site and have registered on their mailing list, in the hope they have some special offers. I think it said their timetable for next year is due out round about November.
If you google Brittany Ferries the website should come up.
I'm also keen to hear if anyone knows of a better/cheaper way of doing it,
Lara.