Can you break these bills out by therms per day to normalize them? If you take the gas, used history and divide the therms by the days, you can come up with an average terms per day used. And you can see how much is being driven by there being 40 days in this current bill. That only accounts for a portion of the huge difference, but it does it account for some of it.

Your average therms per day in December 21 was 4.75. In December 22, it was 8.55. If you can approach the gas company with that difference, that very clearly shows that something has changed drastically, that is unrelated to the difference between 28 days in the December bill for 2021 and the 40 days in the December bill for 2022. You could also check the average daily temperature for the billing for each December bill. Document all that and email it to someone at national grid and if I were you, I would also contact your state representative and tell them that you are having trouble resolving an exceptionally high heating bill and let national grid know that you are doing that.