Hi Ciaran,
Unfortunately it is not just about battery capacity. As mentioned by Tony, you also need to account for weight and aerodynamics, etc. With a slight added weight, and less aerodynamics, maybe bigger tyres so higher rolling resistance, etc, could all contribute to lower MPKWH. Any added electrics as well like a cool box, heated seated, etc?
Also, the weather has been a little colder recently, so have you had the heating on more?
I just don't want you to think about comparing apples with apples.
Finally, i do have to say 1.8 MPKWH is quite low.
Thanks, Joe