After flashing from Gingerbread 2.3.4 to JB 4.1.2 the only problem I had was that my 'Lock Screen' and 'Power Menu' had disappeared and no amount of fiddling with the settings would bring them back. Although this forum was invaluable in helping me do the upgrade I couldn't find a solution to this problem here. Eventually found a solution HERE. I posted details of all the steps I took, to upgrade and restore the Lock Screen & Power Menu, to my original 'plea for help' thread which can be found HERE in case it can be of use to someone else. My phone is now running beautifully btw; all the effort was definitely worth it!