You've got a winning appeal!
If you sent reps on 13 July (which you can easily prove if it was by email) then the 56th day from that date is 7 September according to
Add or subtract days, months, or years to or from a specific date.
:

The Notice of Rejection is dated 6 September, so the legally presumed date of service (absent evidence to the contrary) is 8 September, one day out of time. The council contains the date of service is 12 September, and you say it was the 15th. No matter which date you use, they've missed the 56 day deadline which makes it an open-and-shut case.
Would you like me to represent you at the tribunal? If so, drop me a PM please.