Ok Tina thanks for that. So your problem here is:
1. You have no proof of the posting date of the renewal letter - only a 'guess' at the 15th November.
2. In this case the insurance company can insist on going by the date on the letter and since this gives you 20 days from then to cancel under the Loi Chatel, this brought you to the 25th November - which has gone.
3. If you were right about the date stamp on the envelope - date stamp and not the date it was delivered, then you would add 20 days to the 15th November and arrive at the 5th of December - which has also passed!
So not so good really.
My advice, if you have not already done so, is call their bluff, send a cancelation letter under the Loi Chatel and see what happens, always knowing that for this year you are on shifting sands. The only other thing you could do is get someone like me to talk to the insurance company and twist their arm, but not before you have:
a) Sent the cancellation letter in proper form and RAR, and
b) Received a written refusal from the company with their reasons for refusing your cancellation.
Sorry I can't be any more positive or helpful than that in your case.
Lesson for everyone who reads this KEEP THE ENVELOPES!!
Best of luck, Tim