VNC password wont change

Hello all,I am new to using these forums so please be gentle!

Problem: I am trying to set up a password for "screen sharing" on my iMac running Mountain Lion. I click in "computer settings" in the sharing option of screen sharing under system preferences>sharing - there is a blocked out password on there and I type a new one over the top. My mac prompts me for my user password to confirm changes, which I enter successfully and click OK. If I then click into the settings again the password has remained unchanged (I can't see the password, but I can see that it has reverted to the original number of characters and my password has not changed). It doesnt matter how often I try this, the password for VNC screen sharing will not change.

Please Please Please help me with a reason/fix for this...

Thank you in advance