Janet, I have huge ones in France, almost as big as a small tangerine.
They have a whitish/grey shell - can I eat them if I prepare them
properly?

You can eat what you wish, Judith, whether you enjoy it is another
question :-)

. They sound like the edible snail to me; I had one as a pet once (
after a stormy showdown at the petshop. I had just sold a lot of baby
mice (under duress) for enough money to buy a snake, but was denied
permission for the snake by my father. He let me keep the snail
freerange in his greenhouse, though.

(Thinks...whatever did happen to my edible snail? )
Mice, snakes and snails, are you sure that you are not my younger
daughter. Mary went to bed clutching her "pet" snail. She was
distressed in the morning to find that she had lost it - I found it - on
the ceiling.
--
Judith Lea