Felly sgrifennodd John Hall :
"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"!

According to my Oxford English Reference dictionary, the past participle of
eat is eaten. You may say "I was eaten", which is passive (it was done to
me). In the same way, if someone or thing made you sit, you would be sat.

I think.

Actually, I quite like the phrase "I was sat"; it has an archaic feel about
it. It certainly suggests to me that "it was just happening" rather than
"I was actively doing it", which is a useful literary device.

Adrian

--
Adrian Shaw ais@
Adran Cyfrifiadureg, Prifysgol Cymru, aber.
Aberystwyth, Ceredigion, Cymru ac.
http://users.aber.ac.uk/ais uk