Child seats in an '18 C Class cab

[FD3Si]

My wife's just got a 18 plate C class Cab. The rear seats are pretty upright anyways, and the headrests seem non adjustable. I went to put my son's child seat in (Recaro Monza Nova Seatfix) and it sits really, really upright. The backrest is near vertical, even on with the ISOFix rails fully extended. This also means there's a bit gap between seat back and the child seat.
I'm guessing this is far from ideal, I've contacted the manufacturer to check if it's even safe! (Obviously I'm not using it at the moment)
Has anyone experienced similar? And are there any parents out there with youngish children who can recommend a group 2/3 seat that is safe, and doesn't have this issue? ISOfix would be preferred so it doesn't rattle around when the belt's not done up!