FS install on the SL808x has been resolved.
Turns out is was nothing to do with the programming - I had purchased devices running a dated bootloader version, which was preventing the FS install form working. I had upgraded the FW but didn’t realize I need to also upgrade the bootloader.
Broken bootloader version: S4_1_0_7BT R1431 CNSZXD00000155 2012/11/19 16:06:52