Ok, I've just run the automatic test again (repeat every 5 mins x 5 times, default settings). My results were very similar to my last run - consistently <1Mb, with a spike or two distorting the average.
According to general consensus the minimum required speeds for streaming are:
720 - 2.5 Mbps
1080 - 5 Mbps
2k - 10 Mbps
4k - 20Mbps
I've gone to the ‘Costa Rica in 4k’ test stream on YouTube (https://youtu.be/LXb3EKWsInQ?si=M4bTYIg4oLYl09n) - streams fine at 720 and 1080/60fps, stutters considerably at 2K and forget about 4K.
If, according to testmy.net, my true download speed is 600kbps, how can I stream 1080 smoothly?