Thanks very much. As you can guess, I only started looking at web development just over a week ago, so I'm very much a beginner. Your answer explained my mistake very well, thanks!
I managed to get around my problem by using hidden parameters (I'd tried this previously but incorrectly and just assumed that it didn't work) and now my script does what I need it to.
If I start needing to do more web development than I am just now, I'll have a deeper look into other options (but for now this works!)