I'm having my first attempt at installing LFS, and have got as far as Ch.6, and the Glibc section. I've done the configure, make, and make check which appeared to go fine. But when I try make install it gets through quite a lot of it, but then Errors out when trying to copy math.h to /usr/include. The error message it reports is: "read-only filesystem", so can't write the file.

I can't see why it has suddenly become a read-only filesystem halfway through a make install?
I tried copying the file while logged in as host root, but it seems the whole lfs directory structure has become a read-only filesystem. The only way to free it up again is to reboot the host linux.

So I'm rather stuck at the moment!

you can write to the filesystem as root after reboot? doesn't it tell you this 'cause the disc is full?

There's >20 giga-bytes left on the drive, so that's not a problem.