Thank you for such a prompt and correct answer, and I will try and be more precise with error messages etc. in the future.
Yes, my friend was indeed invoking IDLE3 by typing sudo IDLE3 at the terminal, whilst I was invoking it from the GUI desktop.
So this (my first!) post is now closed - thanks again

When I write a program in Python using IDLE3, I can run it from the IDLE3 Python shell. But when I add GPIO.setup(7, GPIO.out) to my code I get an error message 'No access to DEV/MEM Try running as root. To make it run, I have to go to the terminal and type SUDO python filename.py then it works. The...