Ok.
I am English and I always have got this one confused, along with a load of other people. The amount of Enlish people who may say this:
"I was sat on the chair when the TV broke"
Now this is probably incorrect. If you were 'sat' on the chair, then this means that a person personally placed you there (like an usher at the theatre may do). What should be written is:
"I was sitting on the chair when the TV broke"
EDIT: Just to add to this - You could say I sat on the chair when the TV broke as this would mean that you yourself sat down as opposed to someone else sitting you down.
Hope that helps anyone who has gotten confused about the difference before - although if you said it, everyone would understand you, and 99.9% of people would not pick you up about it - although my Dad would!
Andy