So after much head scratching the EEPROM selfboot is working again. The SPI address doesn't seem to matter as long as the EEPROM settings are correct. MP0, MP1, MP2, MP3 pins also need setting correctly - although I have changed these a few times before they are correct as it's not completely obvious.
This all needs documenting properly. Hopefully with this box of tricks now working, almost as expected pending a reduction in analogue output levels, I can start doing that.
Other reworked PCB layouts are still work in progress, a few more tweaks needed to finish them off.
An electronics engineer and a long term electronics hobbyist. I like tinkering with stuff and making things.