Thank you very very much! I'm sorry I didn't realise this wasn't specifically a slime problem - I assumed because it didn't happen from the terminal it had to be specific to slime.
Really really bizarrely, I had clearly fixed this problem in the past, since my .bashrc has a line exporting SBCL_HOME, but for some reason it doesn't get as far as emacs. This is why it was working from the command line. Anyway, (setenv) from emacs makes it work, so the rest is just cleaning up and not your problem.
Thanks again for the help!
Rupert