OK, so it wasn't Afrihost, because my phone was streaming at full 4K without any issues.
I thought a reinstall of the app on my TV would cure it, but when I went to install again, the app was not in the LG Content store anymore, which was very strange.
I eventually found that at some point last week, my TV did a software update, and changed the region from ZA to Other.
Once I had set it back to ZA, the Netflix app was back in the LG Content store, and once installed it was streaming full blast as before.