Thanks everyone. What brought this to my attention was my fitting a split screen to use with manual lenses. I hadn't noticed any problems with the OEM screen but then, I hadn't been using it with manual lenses.
I just checked and I have the screen in correctly.
I use the dioptre adjustment a lot as I have to change it when I flip from left eye to right eye - I'm long-sighted in the left, short-sighted in the right. Oddly enough, when I put the camera to the left eye, it is naturally level. When I lift it to the right eye, the horizon is always wonky.
I'll double check tomorrow to see if the red indicators show up and when.
This image looked perfectly focussed in the viewfinder.