Unhappy IMEI Number mismatch after re-installation of stock rom

Hi everyone,

I have read up a lot on different people in similar situations to myself, but none of the solutions I have found have been successful. Anyway, here goes:

01) I rooted my GT-N7100 (Samsung Galaxy Note II) using Odin and then installed Cyanogenmod onto it.
02) I decided that I did not like CM, so I downloaded the stock rom for the handset and went through the process of putting the phone back to normal
03) This was successful, however, during the process my IMEI number has changed on the OS level, so that now the IMEI written on the device is different to the one that is displayed in the device status.

I am not trying to change my IMEI number to anything other than the number that it is supposed to be, I just want to be able to make calls and use my mobile data again. If anyone has any ideas about how I can go about editing the current IMEI number, I would be really appreciate it. I'm aware that this is a taboo subject due to the legality of changing the number, but all I want to do is get my phone back to the way it was.

Please feel free to ask me for any further information.

Thanks in advance.