Late arrival at Roscoff

I have just booked a crossing to Roscoff via Plymouth and will be arriving at 21.30. We are spending the first part of the holiday in Perros Guirrec which is only about an hour away but probably too late to arrive on site there. Any one know of a late arrivals site or whether it is possible to spend a few hours on the dock in Roscoff ?This was just as cheap as Portsmouth/Caen after adding in Camping Cheques - no point doing the extra miles!