I've used the other fix described in the thread "One slight issue with the browser back button" and it works fine. However there is another issue..
If you reply to a comment the reply normally appears embedded in the parent comment. However if you enter the Really Simple CAPTCHA incorrectly you are taken to a page asking you to press the browser's back button and try again.
After you press back the comments form is no longer embedded, so your comment appears as a new comment with no relationship to the parent comment.
I like the Really Simple CAPTCHA because it's really simple. Hope these minor issues can be resolved.