How can I help with this please?
If you are serving notice today then the end date is the 10th December.
The term date is the 11th. Your notice therefore will not start to run until the 10th October and must run for two months which takes you until December.
Can I clarify anything for you?
Yes, first class post. Although you can always send it recorded delivery.
Yes, even though it is a period you still have to use the S21 notice procedure.
The 11th would be giving them an extra day.