Sounds like yours is a 60/40 split same as my Astra. You can have three seats across a 60/40 split as long a child seat doesn't straddle the split, that's where it is a safety issue. They don't install anchor points directly behind a split, hence only two anchor points in a car with 50/50 split seats.
Thanks melbel that makes sense
I would imagine you'd still need to be mindful of the anchor strap not slipping between, it could not be as tight as needed, or not behave the way it should in an accident, maybe? I haven't had a good look at mine though so it could be far enough away that it won't matter.