The .htaccess file that cPanel generates has extra whitespace at the end of some of the lines, on the four cPanel domains I have or had, when the lines are trimmed back to the last character, it worked for me.
I can't explain why, I can only say that it does.
Robs example would work fine if you created your own but like many people, I initially didn't know about the .htacess file and I wanted it to work from the GUI. I spent a lot of time looking into the problem, cutting and pasting and generating new files, testing etc.
The solution above was what I found worked.
Apologies if the tone in the previous posts was offensive, the inital post did have a

in it and Rob's first post offered a solution but not a cPanel solution which is what people here are looking for.