In article ,
Joe writes:
According to my Longman's dictionary. To say "i was sat outside" is
correct, as "sat" is the past participle of sit, and is a verb.

"Sat" is indeed the past participle, but it isn't used in the so-called
"past imperfect" tense. Thus you could say "I sat" (perfect tense) or "I
was sitting" (past imperfect tense), but not "I was sat". Perhaps
another example will make it clearer. You would say "I ate" or "I was
eating", but surely not "I was ate"!

While "sitting" is a noun where is it use to describe a period when
you are sitting in a chair"

As you say, it is used to describe an action going on over a period,
which is what the past imperfect tense does. My OED also calls "sitting"
a noun, which surprised me. It's the present particle of the verb "to
sit", which I would have thought made it a verb. As well, as "I was
sitting", it's also used in the present tense, where "I am sitting" is
an alternative to "I sit".

Perhaps we ought to raise the question in uk.culture.language.english
--
John Hall "Never play cards with a man called Doc.
Never eat at a place called Mom's.
Never sleep with a woman whose troubles
are worse than your own." Nelson Algren