I almost didn't write this as I don't want to share my secret but as you have all been so helpful to me I thought I would tell you (briefly!) about it. In a nutshell, we loved the area and the campsite and are booking ourselves back in next year with our own tent for 2 weeks. It was wonderful.
We were at Camping La Baie in Trinite sur Mer near Carnac. It's only about a 2 1/2 hour drive from St. Malo or 4 1/2 from Cherbourg.
The Eurocamp option worked really well, there really was nothing we could have needed and were extremely comfortable but I did always look longingly at the people in their own tents and think "I wish I had our own stuff". The Eurocamp pitches weren't the best at the site although they were still fine.
I can strongly recommend the campsite. It is very small but has everything you could need. It is surrounded by beaches, has a good children's play area, a lovely small pool, a shop and a restaurant on the beach (which was a bit pricey) but you could get takeaway pizzas and chips etc for those lazy days. Most important (for me) the shower blocks and everything really was immaculate.
The campsite does put on low key entertainment but its really for children of 5 and upwards. The entertainment is things such as water polo in the pool, aqua-aerobics 3 times a week childrens games etc. If you want peace and quiet though it is not intrusive.
I'm trying to keep this short so I'll finish now. If I could find one negative point and its really not that bad, its that its sooo quiet in the mornings! The children all seem to be up late and the campsite has a "silence" rule from 1030pm until 8am. 8am - I know!!! I was always conscious of my children making noise early on as it really was quiet. Miraculously though that sea air let them sleep until about 7.30 - bliss!
If anybody wants any more info then I'm happy to send links to pictures etc. I'm off now to get browsing for all the stuff I want to buy for next year - I've got the bug!!!