OK, been learning a lot about building OpenWrt. I used the .config file from the linino repository, which sets up the build for the Linux kernel, and hundreds of packages. I ran out of disk space. (> 5GB)
I have realised that only a very small number of these packages actually end up on the Yun, but I don't know yet how these are selected.
For now, I'm going to start again, and at the "Make menuconfig" stage, I'm going to strip put many of the packages, such as multimedia, gnome, etc.
This should save me time and disk space.
If any kind soul who understands the entire build process would care to enlighten me about how the packages that are finally selected for the Yun image are specified, that would be great :)