It’s colloquial. I think the easiest way to understand it is to see as adjectival rather than a mis-use of the past continuous tense, so ‘sat’ in ‘I was sat there doing nothing’ can be seen as based on a past participle just like, say, bored, terrified, excited etc.So it’s like a (grammatically correct) passive sentence such as ‘We were sat in the third row by the usher’. So why would we use a passive form when there is no stated agent I hear you say. Well my theory is that it conveys a slight sense of not being strictly intentional. So ‘I was sitting there’ implies that it was my intention to be sitting there whereas ‘I was sat there’ has more a sense of ‘by chance’.
Alastair Banton ● 2212d