Hi Nigel
the file name is cs757_C01_T.bmp (this file is located for each livery folder).
You can try to make any changes via this one DXT-BMP converter
http://www.avsim.su/jump.phtml?http://www.btinternet.com/~mnwright/download/dxtbmpx.EXE

Open this file by this converter, save it in any other folder (Desktop, for example) in "Extended 32-bit 888-8" format. Then, open this file by standard Microsoft Paint, make changes to new correct a/c registration, save it. Finally, open this file via DXT-BMP converter once again and save it in DXT1 format.
So, put this updated file to the folder back with replace ( don't be forget to make the copy of original file before all actions!)