Hi Calummurray,
please check if you have followed all the instructions in the ReadMe file. Also, have a look at
this manual. It might also be a problem related to the Oblivion.ini file. Open the Nehrim Launcher and go to Settings/Extra. Under "Location of the ini directory", check "Nehrim directory". Click the buttons "Reset Changes (create new INI)" and "Accept". If necessary, check your Data Files (Nehrim.esm, NehrimData.esp and NehrimEnglish.esp) again and start Nehrim. Can you start a new game now? (If it works, you can quit the game and open the Launcher again to adjust your graphic and other settings.)