Sounds reasonable to me too. The most expensive sites can be £60 plus depending on pitch type for a family of 4 (kids 7 or over) during the high high season (i.e. 7 July to 18 Aug -ish). If you're travelling outside those dates,£35-40 per night is a lot more than you would pay using one of the discount schemes like ACSI or Camping Cheques.
Is £35-40/night a resonable price for a campsite in France during high season. We are 2 adults and 2 DCs and need the electic hook up. It does seem like a very well serviced site (pools/bar/activites etc) I'm out of touch with prices for France - this is near paris, not the beach