Robin,
I’m having a very similar problem, not being able to access my WP-admin login after putting in an incorrect code in my functions.php file. So, if I go this route you’ve mentioned, will it restore the same theme I had installed or will I have to go back and do everything over from a default theme, like graphene or something..?
Biggest problem for me now is that my clients hosting company is laughing squid, which I’ve never heard of, and they apparently don’t have any support on the weekends. If it were hosted by go daddy, i’d just have them restore the site.