Get yourself an ACSI card (www.campingcard.com), Les Isles takes the card and you'll pay just 11 euros per night for two people, car and caravan (or tent or motorhome), awning and electricity. The ACSI card is accepted up until the 3rd July, and then again after the 23rd August, indicating that these are their 'low season' so you shouldn't need to book.
However, if you do want to make a reservation there is a form on the Camping Card website that you can complete in English and email to the site.