Gill C and Slowcoach.
I am afraid that slowcoach is out of touch with the Plymouth/Roscoff route. I have used this route since moving to the Southwest, some times twice a year, for the past 20 years.
From this spring the whole timetable is to change. This is to coincide with the launch of Brittany Ferries new luxury ferry the "Armorique" which has been earmarked to be dedicated to the Plymouth/Roscoff route.To Roscoff there are only a couple of daytime sailings per week, premium rates, the rest are night sailings. The great thing about the night sailings is, sail at 2200hrs and dock at local time 0800hrs,9 hours. A good nights sleep on a new luxury ferry. You do not have a long drive to your site and you can be in the middle of the Vendee by lunchtime.
Fortunately for us, our return ferry that we always use is still in the timetable. We stay overnight in St Pol de Leon, do our shopping -wine etc- there are two campsites there, and catch the 0915hrs Sunday sailing, the only day in the week this ferry sails, and we are home early afternoon.This return sailing is on the Brittany Ferries flagship "Pont Aven" the largest ferry in service. Just Google "Brittany Ferries" and you can download timetables and watch a video of the "Armorique". Hope this helps, if not come back.
Good luck. JTS.