Thanks all. Had already discovered and switched over to using esp_partition_mmap().

Solved. I didn't realise the "memory" parameter was an enum for the TYPE of memory being addressed. Thought it was an actual memory address that was to be where the map is located. Insufficient coffee.

esp_err_t spi_flash_mmap(size_t src_addr, size_t size, spi_flash_mmap_memory_t memory, const void **out_ptr, spi_flash_mmap_handle_t *out_handle) Is there a "correct" method of selecting an address as the target of the mmap? i.e. how do I select a value for the "memory" parameter? I have a custom f...