FWIW; I have pretty much stopped watching the M/kWh gauge during this colder weather when running the heater. I don't believe it includes the heater use. It is the current into the motor.
That being said, every once in a while it seems to read worse when I run the heater.
Now, there could well be reasons for that - the battery runs less efficiently when a larger current is drawn. But if the heater is running, then that adds to the traction current drawn and both then become less efficient.
Some other reasons for reduced range may well be that the battery has protections built in when it gets colder, and excludes using more of the extremes of the battery. I deliberately try to avoid full charges, but have been doing that more recently because otherwise the pre-heat doesn't work. But it is entirely possible that, for example, 100% charged is actually 80% 'real' battery capacity, say from 5C to 30C, and outside those bounds the software drops accessible battery capacity to 70%. (just illustrative figures for what it might be doing).
As Paul uses his less frequently than us, it might well be that his is undergoing a more thorough cold soak and getting to lower temps. I imagine that heat generated in my battery on the drive I am about to undertake now will mean there is still a little warmth in the battery tomorrow (because the battery is better isolated in the boot than in most other EVs). However, leave it for a few days and it'll lose all that heat.
The above are just thoughts and I don't know for sure on any of it. The other option is that your battery capacity is declining!!
It's a case of 'when', not 'if', and statistically each of our batteries will degrade at different rates, maybe even some have rouge cells that will cause this to happen early. I don't think there is any reason to believe this, though, until summer time. You can't tell what the heck is happening to the juice in the battery whilst it is being squandered into a resistive heating system!
(I still think 2M/kWh sucks! - I got around 3.5M/kWh today, based on SOC meter, with sufficient heat getting to my feet (as described in the other post)