It's appears however, with days and days of revision behind and indeed in front of me, this passion for the English Language's rule book doesn't seem to be as widely shared as it should be. I have one lecturer in particular who seems to adore the misuse of "which". I can handle the odd "which" or "that" out of place in some of the articles I edit for the University magazine, but this guy is serial offender.
So, in a way to make myself feel a tiny bit better, I'm putting the rule on here in the hope of simply publicising that there is a difference. For those of you out there who don't quite recognise that difference, here's the rule of thumb:
"That" is used for anything that is necessary to the main clause - the main clause doesn't quite make sense without it:
"I threw away the bit of cheese that had gone mouldy."
"Which" is used for any added extras to the main clause - the main clause does make sense without it:
"I threw away the mouldy cheese, which meant we had none."
There are of course some places where you can interchange that or which, but not in the instances described. I hope this helps someone out there - if anything, it certainly helped me from sending an email of blind rage to my lecturer!