o.k. so it seems it might be a design limitation. I spoke with another MG ZS driver who confirmed my findings, which are: 1. If the tailgate is closed, but not closed properly, once you start driving (and presumably as you encounter minor bumps in the road) the door ajar symbol displays. Since I am driving I can't look too closely but I don't think the tailgate is highlighted in any way as the point of fault, unlike with the doors. Further as you travel the the alert disappears, then comes on again (I'm guessing due to minor bumps in the road). However, if you leave the tailgate wide open, it would appear that there are no warnings given, unless you try and lock the doors (or per the earlier comment by peterseddon) if you try to use cruise control.
I have booked the car in with the dealer already, but I might call to ask them to research, since I don't want a wasted service visit.