I was educated in the north of England (many years ago), where there has always been a tendency to say "I was sat on the ....". Teachers placed a great deal of emphasis on the fact that it was NOT correct (same the common northern usage was "borrow/take/etc. off" instead of "from"). Not having lived in England for many years, I am now amazed at how this expression has crept into common use by educated English speakers and in written English. "Sat" is either the active form of the simple past tense ("he sat on the chair", i.e. describing the action of becoming seated) or the past participle ("the child was sat on the chair by her father", i.e. describing the action of one person seating another person -sounds a bit forced but would be grammatically correct-). I know all kinds of things can become common usage and, as I say, I believe "sat" instead of "sitting" has always been used in the north of England, but I really can't find a grammatical leg for it to stand on. I am a translator and I would certainly never use the "I was sat on the chair" form in written English if what I meant was "I was sitting on the chair".