I just saw advice from martinayotte
Posted May 23, 2020
"Rockchip SoCs always have eMMC as boot priority.
So, to boot from SDCard, you need to stop U-Boot from eMMC by pressing <spacebar> several times during startup.
Then, at U-Boot command line, you can type "setenv devnum 1" followed with "run mmc_boot", it will then boot from SD."
I've made a note of this for myself as well, cheers