Thanks for responding so quickly.

I recreated the image and no change to the result. It doesn't appear that any partitions have been created, it still only has the FAT32 RECALBOX partition.

As the FAT32 RECALBOX partition only uses 756Mb of the 3070Mb partition I resized it down to 800Mb and that completed the first boot successfully and now I'm at the main menu (on the ZX Spectrum).

Any reason the FAT32 RECALBOX has so much free space? Other distros seem to ship with a minimal FAT32 partition.

Used Raspberry Pi Imager to image 4Gb microSD card (speed unknown) - booted on RPi0w without modifying config.txt etc.
Connected to 1600x1200@60 Dell 2007FB LCD via mini HDMI to DVI cable, power provided by monitor (powered hub), USB SNES controller, USB headset & wireless USB keyboard & mouse via monitor hub. Config works with Lakka (after removing hdmi_group=1 from config.txt include) and RetroPi 4.5/4.7 (ex sound).

Recalbox booted and eventually the splash screen with the Pacman ghosts appeared. The SD activity light has been flashing for several hours, but I have no idea what it is doing and whether I just need to wait longer or if I start over with a bigger & faster microSD.