I think I have found it...
I can now boot with the uncompressed initramfs, compressed is still a problem but looks to be resolved by the rebase (waiting on your 1-14 label for that).
These were missing: +CONFIG_COMPAT=y +CONFIG_BLK_DEV_LOOP=y +CONFIG_BLK_DEV_RAM=y +CONFIG_BLK_DEV_RAM_SIZE=65536
I tried with COMPAT yesterday but the combination of that and BLK_DEV_RAM seems to have fixed the problem.
Thanks for your help!
Mark