After much investigation I found that the best way is to go through a french insurance broker ... offices in most towns in France.
In the end I decided not to insure at all!
I'm covered when towing via the car and recovery packages and the storage is ultra safe ... plus, I reckon, if something awful happened to the store, then the storer would be liable.
I've paid out on insurance for years ... never needed it ... so I'm giving up ... think the companies who offer it are making money for the proverbial old rope!
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.