I told you my brain had left me.
Quite frankly, I'm embarrassed! I always usually use arrays for groups of checkboxes... what an idiot!
More sherry anyone? *Hic*
Edit: Onion2k, I agree with you about the way checkboxes are grouped though. There is a school of thought that says you shouldn't ever use the value attribute with a checkbox, and that the name is the checked value. (Hence me giving meaningful names). In this case 20 of the checkboxes do relate to a specific subject, so I could get away with the array method. But I wouldn't use this method to check an entire page of checkboxes if they weren't related.
Sorry if it sounded lazy about the if (isset()) thing. I was actually concerned about how the code would look on the page for editting later. I was trying to keep things neat.