I was going to say that I had - I did update the u-boot package - but then I realised that the version number I saw on the serial console was not the same as that in the package description. Long story short, I forgot to actually write the u-boot image to the system disc on ALL the machines I have just switched to current... Interesting that it's only been the lime2 which failed to boot under those circumstances. Anyway, one quick dd session later and everything looks in order again.
Thanks for having pointed me in the right direction!