Hi Hilary - well we've just got back early this morning from a week in Normandy with Brittany Ferries, ferry and cottage booked through them.
We went with our friends who were on a pretty strict budget and I was totally horrified at the cost of the ferry tickets
However, booking our cottage with BF made the tickets cheaper.
I know a lot of English owned-cottages over there have a discount code they can use with most of the ferry companies.
When we were looking back in January, the most expensive ones were those who advertised "discount ferry tickets". Whilst I realise ours was more expensive due to going over May half-term, our friends usually go mid-week out of school hols and have found it much cheaper then . . . something to consider, if you can.
Where abouts are you going? We stayed 5km outside of Granville in Normandy. It's a lovely area. Brilliant time had by all, especially due to all the D-Day anniversary stuff (didn't plan on doing any, but once we'd done the Bayeaux War Musee, we were all hooked, and that included 2 x 11 year olds).