Thanks to Robert Nalson’s excellent guide I have my pico-pi running linux-5,15 pretty much flawlessly. I thought I’d try a linux-6.* kernel, I saw Robert’s repo had a 8.0.*rc branch so I grabbed that, did the config then copied the .config to a freshly unpacker linux-6.2.6 tree. I did a make ARCH=arm CROSS_COMPILE=${CC} oldconfig and got a load of prompts for features and accepted all defaults then build the kernel, dtbs and modules as for Robert’s original guide. I then just copied them over wifi to the pico-pi and essentially mirrored what was there already, just using a kernel version of ‘6.2.6’. I left the 5.15 kernel and modules etc all there since the new kernel components were all in their own directories and then edited the extlinux.conf file to update the version number. Sync-ed the filesystem and rebooted… nothing 
Ok, so I reset the board juspers and got it up in USB mass storage so I could mount the pico fiilesystem and revert the extlinux.conf file then reset the jusmoers and it boots fine.
So this is a rather long winded way of asking what I need to have in the vanilla kernel config in order to be able to boot it? I’d like to be able to compile more recent kernels but it isn’t immediately clear to me what I need. I didn’t have time to try removing the ‘quiet’ option from the boot flags, I guess I should have done that…
Any suggestions or hints welcome…