Hi Pobs,
I eventually took it over to simHQ, where a solution was (almost miraculously!) provided - turns out I had to turn off the PlayIntro file, of all things. Quite why the intro video should interfere with a single end of campaign screen, I've no idea, but there you go. Thanks for your help, it definitely helped put me on the right path to making what is at heart a great game properly playable again. I wish I could say I'd return the favour someday...if I had any tech knowledge to speak of I probably would be able to...
Anyway, cheers again,
Jonny