I must say I've flown around France many times over the last 15 years and landed at private sites (mainly hotels) on I guess 50 occasions over the years. In the past I have never had the landing permit and have never even been asked for one, let alone had a problem. Nevertheless I did think I ought to get one and sent off the forms about 9 months ago. Still haven't heard back!
I may well have just been lucky all the time. Perhaps those who have been asked to produce the permit could let us know!