Well, I just had a look around this 'was sat' thing. 'Sitting' is the present participle of 'sit', so is not 'was sitting' an oxymoron as 'was' implies the past which contradicts the present 'ing' implication?
It looks like one of those cases where common use has mixed two versions of similar things.
"I was sitting waiting for the bus" ... "I sat waiting for the bus"
The context of using those two would (in my mind anyway) be a little different, but by using sat instead of sitting in the first you reduce the number of syllables and letters which makes it quicker to speak or write (ie. tap out on a phone ).
In the first, the 'was' changes the meaning to the past tense, but the second is already in the past tense so does not need the 'was' qualifier.
Reducing the number of syllables does not make it right.
But both the above usages are in the past tense, so either is correct as far as I can see.