Right, I've fixed the issue. I had a search and realised that steam had listed the NTW installation folder under my 'E' drive rather than my 'C' drive. I looked and, sure enough. Napoleon.exe was there. I suppose the NTW folder within my C drive was simply a left over from a previous installation. Anyway, I copied over the file to my C drive and, thank God, the darthmod installer managed to find it.