Thank you again. That's got me a little further.
I notice from looking at the sd card it uses ext3 for the boot partition, ext4 for the root file system, so the dd to the mmc must preserve that. Unfortunately the boot loader only has an ext2load and ext4load. Consulting with a colleague and Mr google, I shouldn't expect an ext3load to be available but ext2load might well load ext3 with a little bit of luck.
I duly proceeded under that assumption and got the following results:
U-Boot# setenv bootargs console=${console} root=LABEL=_/ ro rootwait
U-Boot# saveenv
Saving Environment to MMC...
Writing to MMC(1)... done
U-Boot# ext2load mmcblk 0:1 0x80300000 vmlinuz-4.0.4-301.fc22.armv7hl
mmc_send_cmd: timeout: CTO
mmc_send_cmd: timeout: CTO
5645832 bytes read in 660 ms (8.2 MiB/s)