Me and my trusty sledgehammer deleted all the code to do with the Parmatic parameter for the main camera position, and that worked fine.

But I also went back to the original code and tried the extra test on the presence of materials.
The exclamation mark gives me a syntax error, but it works if I use the syntax " if not materials: ".

Being multi-lingual has its downside, n'est ce pas!

Thank you for your help, I really appreciate it.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)