Not been to site but have used the Portsmouth - St Malo crossing a few times

It is expensive compared to other shorter crossing and on the overnight to St Malo we have always booked a cabin. I have 3 kids and we have crammed the 5 of us into the cabin for 4

I quite enjoy the trip out boat leaves about 20:30 so time for something to eat and a drink on board then go to sleep and wake up at St Malo.

Trip back leaves St Malo at 10:30 and takes all day (if you are going back to St Malo from S. Vendee will need to leave there very early in the morning) , the day does drag but the ferry is large and there is a cinema showing a kids film

thanks, thats useful to know. it will still be much cheaper than the rosyth to zeebrugge crossing which costs a fortune!

i used this overnight crossing this year and and found it great. im not the best of travellers on the water and was dreading the crossing but i actually found it very relaxing, the cabins are comfy with your own shower and loo.
both my boys slept all night[wish they did at home lol] and they loved it. it is more expensive but i would say worth it.